Fix nullability errors.

Radzen.Blazor.Tests/AccordionTests.cs

@@ -123,7 +123,7 @@ namespace Radzen.Blazor.Tests
             });
 
             // Find and click the accordion header link to expand
-            var header = component.Find(".rz-accordion-header button");
+            var header = component.Find(".rz-accordion-header a");
             header.Click();
 
             Assert.True(expandRaised);
@@ -156,7 +156,7 @@ namespace Radzen.Blazor.Tests
             });
 
             // Find and click the accordion header link to collapse
-            var header = component.Find(".rz-accordion-header button");
+            var header = component.Find(".rz-accordion-header a");
             header.Click();
 
             Assert.True(collapseRaised);
@@ -184,7 +184,7 @@ namespace Radzen.Blazor.Tests
             });
 
             // Try to click the disabled item
-            var header = component.Find(".rz-accordion-header button");
+            var header = component.Find(".rz-accordion-header a");
             header.Click();
 
             // Event should not be raised for disabled item

Radzen.Blazor.Tests/DatePickerTests.cs

@@ -155,7 +155,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add<bool>(p => p.AllowClear, true);
             });
 
-            Assert.Contains(@$"<button type=""button"" class=""notranslate rz-dropdown-clear-icon rzi rzi-times""", component.Markup);
+            Assert.Contains(@$"<i class=""notranslate rz-dropdown-clear-icon rzi rzi-times""", component.Markup);
         }
 
         [Fact]

Radzen.Blazor.Tests/FieldsetTests.cs

@@ -152,7 +152,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.Collapse, args => { raised = true; });
             });
 
-            component.Find("legend button").Click();
+            component.Find("a").Click();
 
             Assert.True(raised);
 
@@ -160,7 +160,7 @@ namespace Radzen.Blazor.Tests
 
             component.SetParametersAndRender(parameters => parameters.Add(p => p.Expand, args => { raised = true; }));
 
-            component.Find("legend button").Click();
+            component.Find("a").Click();
         }
 
         [Fact]

Radzen.Blazor.Tests/LoginTests.cs

@@ -119,7 +119,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.AllowResetPassword, true);
             });
 
-            Assert.Contains(@$"Forgot password?</button>", component.Markup);
+            Assert.Contains(@$"Forgot password?</a>", component.Markup);
         }
 
         [Fact]
@@ -134,7 +134,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.ResetPasswordText, "Test");
             });
 
-            Assert.Contains(@$"Test</button>", component.Markup);
+            Assert.Contains(@$"Test</a>", component.Markup);
         }
 
         [Fact]
@@ -215,7 +215,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.ResetPassword, args => { clicked = true; });
             });
 
-            component.Find("button.rz-link").Click();
+            component.Find("a").Click();
 
             Assert.True(clicked);
         }
@@ -234,7 +234,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.ResetPassword, args => { clicked = true; });
             });
 
-            component.Find("button.rz-link").Click();
+            component.Find("a").Click();
 
             Assert.True(clicked);
         }

Radzen.Blazor.Tests/PagerTests.cs

@@ -2,7 +2,6 @@ using Bunit;
 using Bunit.JSInterop;
 using System;
 using System.Collections.Generic;
-using System.Threading.Tasks;
 using Xunit;
 
 namespace Radzen.Blazor.Tests
@@ -55,7 +54,7 @@ namespace Radzen.Blazor.Tests
         }
 
         [Fact]
-        public async Task RadzenPager_Renders_Summary() {
+        public async void RadzenPager_Renders_Summary() {
             using var ctx = new TestContext();
             ctx.JSInterop.Mode = JSRuntimeMode.Loose;
             ctx.JSInterop.SetupModule("_content/Radzen.Blazor/Radzen.Blazor.js");
@@ -65,7 +64,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add<int>(p => p.Count, 100);
                 parameters.Add<bool>(p => p.ShowPagingSummary, true);
             });
-            await component.InvokeAsync(() => component.Instance.GoToPage(2));
+            await component.Instance.GoToPage(2);
             component.Render();
 
             Assert.Contains(@$"rz-pager-summary", component.Markup); 
@@ -112,7 +111,7 @@ namespace Radzen.Blazor.Tests
         }
 
         [Fact]
-        public async Task RadzenPager_First_And_Prev_Buttons_Are_Disabled_When_On_The_First_Page()
+        public async void RadzenPager_First_And_Prev_Buttons_Are_Disabled_When_On_The_First_Page()
         {
             using var ctx = new TestContext();
             ctx.JSInterop.Mode = JSRuntimeMode.Loose;
@@ -124,18 +123,18 @@ namespace Radzen.Blazor.Tests
                 parameters.Add<bool>(p => p.ShowPagingSummary, true);
             });
 
-            await component.InvokeAsync(() => component.Instance.GoToPage(0));
+            await component.Instance.GoToPage(0);
             component.Render();
 
-            var firstPageButton = component.Find("button.rz-pager-first");
+            var firstPageButton = component.Find("a.rz-pager-first");
             Assert.True(firstPageButton.HasAttribute("disabled"));
 
-            var prevPageButton = component.Find("button.rz-pager-prev");
+            var prevPageButton = component.Find("a.rz-pager-prev");
             Assert.True(prevPageButton.HasAttribute("disabled"));
         }
 
         [Fact]
-        public async Task RadzenPager_Last_And_Next_Buttons_Are_Disabled_When_On_The_Last_Page()
+        public async void RadzenPager_Last_And_Next_Buttons_Are_Disabled_When_On_The_Last_Page()
         {
             using var ctx = new TestContext();
             ctx.JSInterop.Mode = JSRuntimeMode.Loose;
@@ -147,13 +146,13 @@ namespace Radzen.Blazor.Tests
                 parameters.Add<bool>(p => p.ShowPagingSummary, true);
             });
 
-            await component.InvokeAsync(() => component.Instance.GoToPage(9));
+            await component.Instance.GoToPage(9);
             component.Render();
 
-            var lastPageButton = component.Find("button.rz-pager-last");
+            var lastPageButton = component.Find("a.rz-pager-last");
             Assert.True(lastPageButton.HasAttribute("disabled"));
 
-            var nextPageButton = component.Find("button.rz-pager-next");
+            var nextPageButton = component.Find("a.rz-pager-next");
             Assert.True(nextPageButton.HasAttribute("disabled"));
         }
     }

Radzen.Blazor.Tests/PanelTests.cs

@@ -167,7 +167,7 @@ namespace Radzen.Blazor.Tests
                 parameters.Add(p => p.Collapse, args => { raised = true; });
             });
 
-            component.Find("button.rz-panel-titlebar-toggler").Click();
+            component.Find("a").Click();
 
             Assert.True(raised);
 
@@ -175,7 +175,7 @@ namespace Radzen.Blazor.Tests
 
             component.SetParametersAndRender(parameters => parameters.Add(p => p.Expand, args => { raised = true; }));
 
-            component.Find("button.rz-panel-titlebar-toggler").Click();
+            component.Find("a").Click();
         }
 
         [Fact]

Radzen.Blazor.Tests/QueryableExtensionTests.cs

@@ -328,7 +328,7 @@ namespace Radzen.Blazor.Tests
             
             var result = data.Where(filters, LogicalFilterOperator.And, FilterCaseSensitivity.Default).ToList();
             
-            Assert.Equal(1, result.Count);
+            Assert.Single(result);
             Assert.Equal("Eve", result[0].Name);
         }
 

Radzen.Blazor.Tests/Spreadsheet/AggregateFunctionTests.cs

@@ -0,0 +1,49 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class AggregateFunctionTests
+{
+    readonly Sheet sheet = new(15, 5);
+
+    void SeedWithErrors()
+    {
+        sheet.Cells["A1"].Formula = "=A2/0"; // #DIV/0!
+        sheet.Cells["A2"].Value = 82;
+        sheet.Cells["A3"].Value = 72;
+        sheet.Cells["A4"].Value = 65;
+        sheet.Cells["A5"].Value = 30;
+        sheet.Cells["A6"].Value = 95;
+        sheet.Cells["A7"].Formula = "=0/0"; // #DIV/0!
+        sheet.Cells["A8"].Value = 63;
+        sheet.Cells["A9"].Value = 31;
+        sheet.Cells["A10"].Value = 53;
+        sheet.Cells["A11"].Value = 96;
+    }
+
+    [Fact]
+    public void ShouldComputeMaxIgnoringErrors()
+    {
+        SeedWithErrors();
+        sheet.Cells["B1"].Formula = "=AGGREGATE(4,6,A1:A11)"; // MAX ignoring errors
+        Assert.Equal(96d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldComputeLargeIgnoringErrors()
+    {
+        SeedWithErrors();
+        sheet.Cells["B1"].Formula = "=AGGREGATE(14,6,A1:A11,3)"; // LARGE k=3 ignoring errors
+        Assert.Equal(82d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorWhenKMissingForSmall()
+    {
+        SeedWithErrors();
+        sheet.Cells["B1"].Formula = "=AGGREGATE(15,6,A1:A11)"; // SMALL requires k
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/AndFunctionTests.cs

@@ -0,0 +1,160 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class AndFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithAllTrueValues()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithOneFalseValue()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(false, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithAllFalseValues()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(false, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithNumericValues()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=AND(A1>1,A2<100)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithZeroAsFalse()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Value = 1;
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(false, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithNonZeroAsTrue()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithStringValues()
+    {
+        sheet.Cells["A1"].Value = "test";
+        sheet.Cells["A2"].Value = "hello";
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithEmptyStringAsFalse()
+    {
+        sheet.Cells["A2"].Value = "hello";
+        sheet.Cells["A3"].Formula = "=AND(A1,A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Formula = "=AND(A1,A2,A3)";
+
+        Assert.Equal(true, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithOneFalseInMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Formula = "=AND(A1,A2,A3)";
+
+        Assert.Equal(false, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForEmptyAndFunction()
+    {
+        sheet.Cells["A1"].Formula = "=AND()";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithRangeExpression()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Formula = "=AND(A1:A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionWithMixedTypes()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = "3";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Formula = "=AND(A1,A2,A3)";
+
+        Assert.Equal(true, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionInIfStatement()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=IF(AND(A1>1,A2<100),A1,\"Out of range\")";
+
+        Assert.Equal(5d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionInIfStatementWithFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 150;
+        sheet.Cells["A3"].Formula = "=IF(AND(A1>1,A2<100),A1,\"Out of range\")";
+
+        Assert.Equal("Out of range", sheet.Cells["A3"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/AutoFilterTests.cs

@@ -0,0 +1,105 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class AutoFilterTests
+{
+    private readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void Should_ToggleSheetAutoFilter()
+    {
+        // Initially no auto filter
+        Assert.Null(sheet.AutoFilter);
+
+        // Apply auto filter to range A1:C5
+        var range = RangeRef.Parse("A1:C5");
+        var command = new SheetAutoFilterCommand(sheet, range);
+        command.Execute();
+
+        // Auto filter should be applied
+        Assert.NotNull(sheet.AutoFilter);
+        Assert.Equal(range, sheet.AutoFilter.Range);
+
+        // Undo the command
+        command.Unexecute();
+
+        // Auto filter should be removed
+        Assert.Null(sheet.AutoFilter);
+    }
+
+    [Fact]
+    public void Should_ToggleDataTableFilterButton()
+    {
+        // Add a data table
+        var range = RangeRef.Parse("A1:C5");
+        sheet.AddTable(range);
+
+        var table = sheet.Tables[0];
+        
+        // Initially ShowFilterButton should be true
+        Assert.True(table.ShowFilterButton);
+
+        // Toggle filter button off
+        var command = new TableFilterCommand(sheet, 0);
+        command.Execute();
+
+        // ShowFilterButton should be false
+        Assert.False(table.ShowFilterButton);
+
+        // Undo the command
+        command.Unexecute();
+
+        // ShowFilterButton should be true again
+        Assert.True(table.ShowFilterButton);
+    }
+
+    [Fact]
+    public void Should_HandleMultipleDataTables()
+    {
+        // Add two data tables
+        sheet.AddTable(RangeRef.Parse("A1:C5"));
+        sheet.AddTable(RangeRef.Parse("E1:G5"));
+
+        var table1 = sheet.Tables[0];
+        var table2 = sheet.Tables[1];
+
+        // Initially both should have ShowFilterButton = true
+        Assert.True(table1.ShowFilterButton);
+        Assert.True(table2.ShowFilterButton);
+
+        // Toggle filter button for first data table
+        var command1 = new TableFilterCommand(sheet, 0);
+        command1.Execute();
+
+        // Only first data table should be affected
+        Assert.False(table1.ShowFilterButton);
+        Assert.True(table2.ShowFilterButton);
+
+        // Toggle filter button for second data table
+        var command2 = new TableFilterCommand(sheet, 1);
+        command2.Execute();
+
+        // Both should be affected
+        Assert.False(table1.ShowFilterButton);
+        Assert.False(table2.ShowFilterButton);
+
+        // Undo second command
+        command2.Unexecute();
+
+        // Only second data table should be restored
+        Assert.False(table1.ShowFilterButton);
+        Assert.True(table2.ShowFilterButton);
+    }
+
+    [Fact]
+    public void Should_HandleInvalidDataTableIndex()
+    {
+        // Try to toggle filter button for non-existent data table
+        var command = new TableFilterCommand(sheet, 0);
+        
+        // Should not throw exception
+        var result = command.Execute();
+        Assert.True(result);
+    }
+} 

Radzen.Blazor.Tests/Spreadsheet/AverageFunctionTests.cs

@@ -0,0 +1,114 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class AverageFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionWithTwoArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=AVERAGE(A1,A2)";
+
+        Assert.Equal(12.5, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionWithEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A3"].Formula = "=AVERAGE(A1,A2)";
+
+        Assert.Equal(10.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=AVERAGE(A1,A2,A3)";
+
+        Assert.Equal(15.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnDiv0ErrorForEmptyAverageFunction()
+    {
+        sheet.Cells["A1"].Formula = "=AVERAGE()";
+
+        Assert.Equal(CellError.Div0, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnDiv0ErrorForAverageFunctionWithNoNumericValues()
+    {
+        sheet.Cells["A1"].Value = "text";
+        sheet.Cells["A2"].Value = "";
+        sheet.Cells["A3"].Formula = "=AVERAGE(A1,A2)";
+
+        Assert.Equal(CellError.Div0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionWithRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=AVERAGE(A1:A2)";
+
+        Assert.Equal(12.5, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionWithMixedTypes()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15.5;
+        sheet.Cells["A3"].Formula = "=AVERAGE(A1,A2)";
+
+        Assert.Equal(12.75, sheet.Cells["A3"].Value);
+
+        sheet.Cells["A4"].Value = 2.5;
+        sheet.Cells["A5"].Formula = "=AVERAGE(A4,A1)";
+
+        Assert.Equal(6.25, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionIgnoringTextAndLogicalValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Value = 20;
+        sheet.Cells["A5"].Formula = "=AVERAGE(A1,A2,A3,A4)";
+
+        Assert.Equal(15.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAverageFunctionIncludingZeroValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=AVERAGE(A1,A2,A3)";
+
+        Assert.Equal(10.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenAverageRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=AVERAGE(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/CellSelectionTests.cs

@@ -0,0 +1,261 @@
+using Bunit;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class CellSelectionTests : TestContext
+{
+    private readonly Sheet sheet = new (4,4);
+
+    [Fact]
+    public void CellSelection_RendersWithCorrectClasses()
+    {
+        // Arrange
+        var cell = new CellRef(0, 0);
+        sheet.Selection.Select(new RangeRef(cell, cell));
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, cell)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-cell");
+        Assert.NotNull(element);
+        Assert.Contains("rz-spreadsheet-selection-cell", element.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-top", element.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", element.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", element.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", element.ClassName);
+    }
+
+    [Fact]
+    public void CellSelection_AppliesFrozenColumnClass()
+    {
+        // Arrange
+        var cell = new CellRef(0, 0);
+        sheet.Columns.Frozen = 1;
+        sheet.Selection.Select(new RangeRef(cell, cell));
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, cell)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-cell");
+        Assert.Contains("rz-spreadsheet-frozen-column", element.ClassName);
+    }
+
+    [Fact]
+    public void CellSelection_AppliesFrozenRowClass()
+    {
+        // Arrange
+        var cell = new CellRef(0, 0);
+        var context = new MockVirtualGridContext();
+        sheet.Rows.Frozen = 1;
+        sheet.Selection.Select(new RangeRef(cell, cell));
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, cell)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-cell");
+        Assert.Contains("rz-spreadsheet-frozen-row", element.ClassName);
+    }
+
+    [Fact]
+    public void CellSelection_CalculatesStyle()
+    {
+        // Arrange
+        var cell = new CellRef(0, 0);
+        sheet.Selection.Select(new RangeRef(cell, cell));
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, cell)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-cell");
+        Assert.Equal("transform: translate(0px, 0px); width: 100px; height: 24px", element.GetAttribute("style"));
+    }
+
+    [Fact]
+    public void CellSelection_SplitsMergedCell_WhenIntersectingFrozenRow()
+    {
+        // Arrange
+        sheet.Rows.Frozen = 1;
+        var range = new RangeRef(new CellRef(0, 0), new CellRef(2, 0));
+        sheet.MergedCells.Add(range);
+        sheet.Selection.Select(range);
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, new CellRef(0, 0))
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var elements = cut.FindAll(".rz-spreadsheet-selection-cell");
+        Assert.Equal(2, elements.Count);
+
+        var frozen = cut.Find(".rz-spreadsheet-frozen-row");
+
+        Assert.NotNull(frozen);
+        
+        // First element (frozen)
+        Assert.Equal("transform: translate(0px, 0px); width: 100px; height: 24px", frozen.GetAttribute("style"));
+        Assert.Contains("rz-spreadsheet-selection-cell-top", frozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", frozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", frozen.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-bottom", frozen.ClassName);
+
+        var unfrozen = elements.Where(e => e != frozen).FirstOrDefault();
+
+        Assert.NotNull(unfrozen);
+        
+        // Second element (non-frozen)
+        Assert.Equal("transform: translate(0px, 24px); width: 100px; height: 48px", unfrozen.GetAttribute("style"));
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-top", unfrozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", unfrozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", unfrozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", unfrozen.ClassName);
+    }
+
+    [Fact]
+    public void CellSelection_SplitsMergedCell_WhenIntersectingFrozenColumn()
+    {
+        // Arrange
+        sheet.Columns.Frozen = 1;
+        var range = new RangeRef(new CellRef(0, 0), new CellRef(0, 2));
+        sheet.MergedCells.Add(range);
+        sheet.Selection.Select(range);
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, new CellRef(0, 0))
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var elements = cut.FindAll(".rz-spreadsheet-selection-cell");
+        Assert.Equal(2, elements.Count);
+
+        var frozen = cut.Find(".rz-spreadsheet-frozen-column");
+
+        Assert.NotNull(frozen);
+        
+        // First element (frozen)
+        Assert.Contains("rz-spreadsheet-frozen-column", frozen.ClassName);
+        Assert.Equal("transform: translate(0px, 0px); width: 100px; height: 24px", frozen.GetAttribute("style"));
+        Assert.Contains("rz-spreadsheet-selection-cell-top", frozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", frozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", frozen.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-right", frozen.ClassName);
+        
+        var unfrozen = elements.Where(e => e != frozen).FirstOrDefault();
+
+        Assert.NotNull(unfrozen);
+        // Second element (non-frozen)
+        Assert.Equal("transform: translate(100px, 0px); width: 200px; height: 24px", unfrozen.GetAttribute("style"));
+        Assert.Contains("rz-spreadsheet-selection-cell-top", unfrozen.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-left", unfrozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", unfrozen.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", unfrozen.ClassName);
+    }
+
+    [Fact]
+    public void CellSelection_SplitsMergedCell_WhenIntersectingBothFrozen()
+    {
+        // Arrange
+        sheet.Rows.Frozen = 1;
+        sheet.Columns.Frozen = 1;
+        var range = new RangeRef(new CellRef(0, 0), new CellRef(2, 2));
+        sheet.MergedCells.Add(range);
+        sheet.Selection.Select(range);
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<CellSelection>(parameters => parameters
+            .Add(p => p.Cell, new CellRef(0, 0))
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var elements = cut.FindAll(".rz-spreadsheet-selection-cell");
+        Assert.Equal(4, elements.Count);
+
+        // Top-left element (both frozen)
+        var both = cut.Find(".rz-spreadsheet-frozen-row.rz-spreadsheet-frozen-column");
+        Assert.NotNull(both);
+        Assert.Contains("rz-spreadsheet-frozen-row", both.ClassName);
+        Assert.Contains("rz-spreadsheet-frozen-column", both.ClassName);
+        Assert.Equal("transform: translate(0px, 0px); width: 100px; height: 24px", both.GetAttribute("style"));
+        Assert.Contains("rz-spreadsheet-selection-cell-top", both.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", both.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-bottom", both.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-right", both.ClassName);
+        
+        // Bottom-left element (column frozen)
+        var frozenColumn = cut.Find(".rz-spreadsheet-frozen-column:not(.rz-spreadsheet-frozen-row)");
+        Assert.NotNull(frozenColumn);
+        Assert.DoesNotContain("rz-spreadsheet-frozen-row", frozenColumn.ClassName);
+        Assert.Contains("rz-spreadsheet-frozen-column", frozenColumn.ClassName);
+        Assert.Equal("transform: translate(0px, 24px); width: 100px; height: 48px", frozenColumn.GetAttribute("style"));
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-top", frozenColumn.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-left", frozenColumn.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", frozenColumn.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-right", frozenColumn.ClassName);
+        
+        // Top-right element (row frozen)
+        var frozenRow = cut.Find(".rz-spreadsheet-frozen-row:not(.rz-spreadsheet-frozen-column)");
+        Assert.NotNull(frozenRow);
+        Assert.Contains("rz-spreadsheet-frozen-row", frozenRow.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-frozen-column", frozenRow.ClassName);
+        Assert.Equal("transform: translate(100px, 0px); width: 200px; height: 24px", frozenRow.GetAttribute("style"));
+        Assert.Contains("rz-spreadsheet-selection-cell-top", frozenRow.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-left", frozenRow.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-bottom", frozenRow.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", frozenRow.ClassName);
+        
+        // Bottom-right element (neither frozen)
+        var neither = elements.FirstOrDefault(e => e != both && e != frozenColumn && e != frozenRow);
+        Assert.NotNull(neither);
+        Assert.DoesNotContain("rz-spreadsheet-frozen-row", neither.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-frozen-column", neither.ClassName);
+        Assert.Equal("transform: translate(100px, 24px); width: 200px; height: 48px", neither.GetAttribute("style"));
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-top", neither.ClassName);
+        Assert.DoesNotContain("rz-spreadsheet-selection-cell-left", neither.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-bottom", neither.ClassName);
+        Assert.Contains("rz-spreadsheet-selection-cell-right", neither.ClassName);
+    }
+}
+
+public class MockVirtualGridContext : IVirtualGridContext
+{
+    private readonly Dictionary<(int Row, int Column), PixelRectangle> rectangle = [];
+
+    public void SetupRectangle(int row, int column, PixelRectangle rectangle)
+    {
+        this.rectangle[(row, column)] = rectangle;
+    }
+
+    public PixelRectangle GetRectangle(int row, int column) => throw new NotImplementedException();
+
+    public PixelRectangle GetRectangle(int top, int left, int bottom, int right) => new(new (left * 100, (right + 1) * 100), new (top*24, (bottom + 1)*24));
+} 

Radzen.Blazor.Tests/Spreadsheet/CellStoreTests.cs

@@ -0,0 +1,69 @@
+using System;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class CellStoreTests
+{
+    readonly CellStore cellStore = new(new Sheet(5, 5));
+
+    [Fact]
+    public void CellStore_ShouldReturnNewCell_WhenCellDoesNotExist()
+    {
+        var cell = cellStore[0, 0];
+
+        Assert.NotNull(cell);
+    }
+
+    [Fact]
+    public void CellStore_ShouldThrowArgumentOutOfRangeException_WhenRowExceedsMax()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => cellStore[5, 0]);
+    }
+
+    [Fact]
+    public void CellStore_ShouldThrowArgumentOutOfRangeException_WhenColumnExceedsMax()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => cellStore[0, 5]);
+    }
+
+    [Fact]
+    public void CellStore_ShouldReturnExistingCell_WhenCellExists()
+    {
+        var expectedCell = new Cell(cellStore.Sheet, new CellRef(0, 0));
+        cellStore[0, 0] = expectedCell;
+        var cell = cellStore[0, 0];
+        Assert.Same(expectedCell, cell);
+    }
+
+    [Fact]
+    public void CellStore_ShouldReturnExistingCell_ViaA1Notation()
+    {
+        var expectedCell = new Cell(cellStore.Sheet, new CellRef(0, 0));
+
+        cellStore[0, 0] = expectedCell;
+
+        var cell = cellStore["A1"];
+
+        Assert.Same(expectedCell, cell);
+    }
+
+    [Fact]
+    public void CellStore_ShouldThrowException_WhenInvalidA1Notation()
+    {
+        Assert.Throws<ArgumentException>(() => cellStore["Invalid"]);
+    }
+
+    [Fact]
+    public void CellStore_ShouldSupport_MultipleLettersInA1Notation()
+    {
+        var cellStore = new CellStore(new Sheet(5, 30));
+        var expectedCell = new Cell(cellStore.Sheet, new CellRef(0, 26));
+
+        cellStore[0, 26] = expectedCell;
+
+        var cell = cellStore["AA1"];
+
+        Assert.Same(expectedCell, cell);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/ChooseFunctionTests.cs

@@ -0,0 +1,34 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ChooseFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldPickScalarByIndex()
+    {
+        sheet.Cells["A1"].Formula = "=CHOOSE(3,\"Wide\",115,\"world\",8)";
+        Assert.Equal("world", sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldPickCellReferenceByIndex()
+    {
+        sheet.Cells["A2"].Value = "1st";
+        sheet.Cells["A3"].Value = "2nd";
+        sheet.Cells["A4"].Value = "3rd";
+        sheet.Cells["A5"].Value = "Finished";
+
+        sheet.Cells["B1"].Formula = "=CHOOSE(2,A2,A3,A4,A5)";
+        Assert.Equal("2nd", sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorWhenIndexOutOfRange()
+    {
+        sheet.Cells["A1"].Formula = "=CHOOSE(5,1,2,3)";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/ColumnFunctionTests.cs

@@ -0,0 +1,40 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ColumnFunctionTests
+{
+    [Fact]
+    public void Column_OmittedReference_ReturnsCurrentColumn()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["C10"].Formula = "=COLUMN()";
+        Assert.Equal(3d, sheet.Cells["C10"].Data.Value);
+    }
+
+    [Fact]
+    public void Column_SingleCellReference_ReturnsThatColumn()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["A1"].Formula = "=COLUMN(C10)";
+        Assert.Equal(3d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Column_RangeReference_SingleRow_ReturnsLeftmostColumn()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["B2"].Formula = "=COLUMN(C10:E10)";
+        Assert.Equal(3d, sheet.Cells["B2"].Data.Value);
+    }
+
+    [Fact]
+    public void Column_RangeReference_MultiRowAndColumn_IsError()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["B2"].Formula = "=COLUMN(C10:D20)";
+        Assert.Equal(CellError.Value, sheet.Cells["B2"].Data.Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/ColumnsFunctionTests.cs

@@ -0,0 +1,30 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ColumnsFunctionTests
+{
+    [Fact]
+    public void Columns_Range_ReturnsColumnCount()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=COLUMNS(C1:E4)";
+        Assert.Equal(3d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Columns_SingleCell_ReturnsOne()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=COLUMNS(C10)";
+        Assert.Equal(1d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Columns_SingleColumnRange_ReturnsOne()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=COLUMNS(C10:C20)";
+        Assert.Equal(1d, sheet.Cells["A1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/ConcatFunctionTests.cs

@@ -0,0 +1,44 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ConcatFunctionTests
+{
+    [Fact]
+    public void Concat_Literals_Works()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["A1"].Formula = "=CONCAT(\"The\",\" \",\"sun\",\" \",\"will\",\" \",\"come\",\" \",\"up\",\" \",\"tomorrow.\")";
+        Assert.Equal("The sun will come up tomorrow.", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Concat_SingleRange_LinearizesRowMajor()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["B2"].Value = "a1";
+        sheet.Cells["C2"].Value = "b1";
+        sheet.Cells["B3"].Value = "a2";
+        sheet.Cells["C3"].Value = "b2";
+        sheet.Cells["B4"].Value = "a4";
+        sheet.Cells["C4"].Value = "b4";
+        sheet.Cells["B5"].Value = "a5";
+        sheet.Cells["C5"].Value = "b5";
+        sheet.Cells["B6"].Value = "a6";
+        sheet.Cells["C6"].Value = "b6";
+        sheet.Cells["B7"].Value = "a7";
+        sheet.Cells["C7"].Value = "b7";
+        sheet.Cells["A1"].Formula = "=CONCAT(B2:C7)";
+        Assert.Equal("a1b1a2b2a4b4a5b5a6b6a7b7", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Concat_MixedArgs_RangeAndLiterals()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["B2"].Value = "Andreas";
+        sheet.Cells["C2"].Value = "Hauser";
+        sheet.Cells["A1"].Formula = "=CONCAT(B2,\" \",C2)";
+        Assert.Equal("Andreas Hauser", sheet.Cells["A1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/CountAllFunctionTests.cs

@@ -0,0 +1,150 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class CountAllFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithTwoArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=COUNTA(A1,A2)";
+
+        Assert.Equal(2.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A3"].Formula = "=COUNTA(A1,A2)";
+
+        Assert.Equal(1.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=COUNTA(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroForEmptyCountaFunction()
+    {
+        sheet.Cells["A1"].Formula = "=COUNTA()";
+
+        Assert.Equal(0.0, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=COUNTA(A1:A2)";
+
+        Assert.Equal(2.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionIncludingTextValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=COUNTA(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionIncludingLogicalValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Value = false;
+        sheet.Cells["A4"].Value = 20;
+        sheet.Cells["A5"].Formula = "=COUNTA(A1,A2,A3,A4)";
+
+        Assert.Equal(4.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionIncludingEmptyStrings()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "";
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=COUNTA(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionIgnoringTrulyEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A4"].Value = 20;
+        sheet.Cells["A5"].Formula = "=COUNTA(A1,A2,A3,A4)";
+
+        Assert.Equal(3.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithAllNumericValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["A4"].Formula = "=COUNTA(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldShowDifferenceBetweenCountAndCounta()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Value = "";
+        sheet.Cells["A5"].Value = 20;
+        sheet.Cells["B1"].Formula = "=COUNT(A1,A2,A3,A4,A5)";
+        sheet.Cells["B2"].Formula = "=COUNTA(A1,A2,A3,A4,A5)";
+
+        Assert.Equal(3.0, sheet.Cells["B1"].Value);
+        Assert.Equal(5.0, sheet.Cells["B2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountaFunctionWithMixedTypes()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Value = "";
+        sheet.Cells["A5"].Value = 3.14;
+        sheet.Cells["B1"].Formula = "=COUNTA(A1,A2,A3,A4,A5)";
+
+        Assert.Equal(5.0, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenCountaRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=COUNTA(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/CountFunctionTests.cs

@@ -0,0 +1,139 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class CountFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithTwoArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=COUNT(A1,A2)";
+
+        Assert.Equal(2.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A3"].Formula = "=COUNT(A1,A2)";
+
+        Assert.Equal(1.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=COUNT(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroForEmptyCountFunction()
+    {
+        sheet.Cells["A1"].Formula = "=COUNT()";
+
+        Assert.Equal(0.0, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15;
+        sheet.Cells["A3"].Formula = "=COUNT(A1:A2)";
+
+        Assert.Equal(2.0, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithMixedTypes()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 15.5;
+        sheet.Cells["A3"].Formula = "=COUNT(A1,A2)";
+
+        Assert.Equal(2.0, sheet.Cells["A3"].Value);
+
+        sheet.Cells["A4"].Value = 2.5;
+        sheet.Cells["A5"].Formula = "=COUNT(A4,A1)";
+
+        Assert.Equal(2.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionIncludingLogicalValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Value = false;
+        sheet.Cells["A4"].Value = 20;
+        sheet.Cells["A5"].Formula = "=COUNT(A1,A2,A3,A4)";
+
+        Assert.Equal(4.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionIncludingTextRepresentationsOfNumbers()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "15";
+        sheet.Cells["A3"].Value = "text";
+        sheet.Cells["A4"].Value = "3.14";
+        sheet.Cells["A5"].Formula = "=COUNT(A1,A2,A3,A4)";
+
+        Assert.Equal(3.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionIgnoringTextAndEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = "";
+        sheet.Cells["A4"].Value = 20;
+        sheet.Cells["A5"].Formula = "=COUNT(A1,A2,A3,A4)";
+
+        Assert.Equal(2.0, sheet.Cells["A5"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionIncludingZeroValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Value = 20;
+        sheet.Cells["A4"].Formula = "=COUNT(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateCountFunctionWithAllNumericValues()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["A4"].Formula = "=COUNT(A1,A2,A3)";
+
+        Assert.Equal(3.0, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenCountRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=COUNT(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/DayFunctionTests.cs

@@ -0,0 +1,32 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class DayFunctionTests
+{
+    [Fact]
+    public void Day_FromDateSerial_ReturnsDay()
+    {
+        var sheet = new Sheet(10, 10);
+        // Using DATEVALUE via VALUE on a date string to get a serial
+        sheet.Cells["A1"].Formula = "=DAY(VALUE(\"2011-04-15\"))";
+        Assert.Equal(15, sheet.Cells["A1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Day_FromDateValue_ReturnsDay()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2011, 4, 15));
+        sheet.Cells["B1"].Formula = "=DAY(A1)";
+        Assert.Equal(15, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Day_InvalidText_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=DAY(\"abc\")";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/DeleteRowColumnTests.cs

@@ -0,0 +1,81 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class DeleteRowColumnTests
+{
+    [Fact]
+    public void DeleteColumn_ShiftsDataAndDecreasesColumnCount()
+    {
+        var sheet = new Sheet(3, 4);
+        sheet.Cells[0, 0].Value = "A";
+        sheet.Cells[0, 1].Value = "B";
+        sheet.Cells[0, 2].Value = "C";
+        sheet.Cells[0, 3].Value = "D";
+
+        sheet.DeleteColumn(1); // delete column B
+
+        Assert.Equal(3, sheet.ColumnCount);
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("C", sheet.Cells[0, 1].Value);
+        Assert.Equal("D", sheet.Cells[0, 2].Value);
+    }
+
+    [Fact]
+    public void DeleteRow_ShiftsDataAndDecreasesRowCount()
+    {
+        var sheet = new Sheet(4, 2);
+        sheet.Cells[0, 0].Value = "R1";
+        sheet.Cells[1, 0].Value = "R2";
+        sheet.Cells[2, 0].Value = "R3";
+        sheet.Cells[3, 0].Value = "R4";
+
+        sheet.DeleteRow(1); // delete row 2
+
+        Assert.Equal(3, sheet.RowCount);
+        Assert.Equal("R1", sheet.Cells[0, 0].Value);
+        Assert.Equal("R3", sheet.Cells[1, 0].Value);
+        Assert.Equal("R4", sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void DeleteColumn_DoesNotAdjustFormulas_RefsBecomeError()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.Cells[0, 0].Value = 1;   // A1
+        sheet.Cells[0, 1].Value = 2;   // B1
+        sheet.Cells[0, 2].Value = 3;   // C1
+
+        // Formula in B2 references A1 and C1
+        sheet.Cells[1, 1].Formula = "=A1+C1";
+        Assert.Equal(4d, sheet.Cells[1, 1].Value);
+
+        // Delete referenced column A -> A1 becomes invalid => #REF!
+        sheet.DeleteColumn(0);
+
+        Assert.Equal(CellError.Ref, sheet.Cells[1, 0].Value);
+        Assert.Equal("=#REF!+C1", sheet.Cells[1, 0].Formula);
+    }
+
+    [Fact]
+    public void DeleteRow_DoesNotAdjustFormulas_RefsBecomeError()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.Cells[0, 0].Value = 1;   // A1
+        sheet.Cells[1, 0].Value = 2;   // A2
+        sheet.Cells[2, 0].Value = 3;   // A3
+
+        // Formula in B2 references A1 and A3
+        sheet.Cells[1, 1].Formula = "=A1+A3";
+        Assert.Equal(4d, sheet.Cells[1, 1].Value);
+
+        // Delete referenced row 1 -> A1 becomes invalid => #REF!
+        sheet.DeleteRow(0);
+
+        Assert.Equal(CellError.Ref, sheet.Cells[0, 1].Value);
+        Assert.Equal("=#REF!+A3", sheet.Cells[0, 1].Formula);
+    }
+
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/FilterCommandTests.cs

@@ -0,0 +1,146 @@
+using System;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class FilterCommandTests
+{
+    private readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void Should_AddFilterWithCommand()
+    {
+        // Initially no filters
+        Assert.Empty(sheet.Filters);
+
+        // Create a filter
+        var filter = new SheetFilter(
+            new EqualToCriterion { Column = 0, Value = "Test" },
+            RangeRef.Parse("A1:A5")
+        );
+
+        // Execute the command
+        var command = new FilterCommand(sheet, filter);
+        var result = command.Execute();
+
+        // Command should succeed
+        Assert.True(result);
+
+        // Filter should be added
+        Assert.Single(sheet.Filters);
+        Assert.Contains(filter, sheet.Filters);
+    }
+
+    [Fact]
+    public void Should_UndoFilterCommand()
+    {
+        // Initially no filters
+        Assert.Empty(sheet.Filters);
+
+        // Create a filter
+        var filter = new SheetFilter(
+            new EqualToCriterion { Column = 0, Value = "Test" },
+            RangeRef.Parse("A1:A5")
+        );
+
+        // Execute the command
+        var command = new FilterCommand(sheet, filter);
+        command.Execute();
+
+        // Filter should be added
+        Assert.Single(sheet.Filters);
+
+        // Undo the command
+        command.Unexecute();
+
+        // Filter should be removed
+        Assert.Empty(sheet.Filters);
+    }
+
+    [Fact]
+    public void Should_WorkWithUndoRedoStack()
+    {
+        // Initially no filters
+        Assert.Empty(sheet.Filters);
+
+        // Create a filter
+        var filter = new SheetFilter(
+            new EqualToCriterion { Column = 0, Value = "Test" },
+            RangeRef.Parse("A1:A5")
+        );
+
+        // Execute the command through the undo/redo stack
+        var command = new FilterCommand(sheet, filter);
+        var result = sheet.Commands.Execute(command);
+
+        // Command should succeed
+        Assert.True(result);
+
+        // Filter should be added
+        Assert.Single(sheet.Filters);
+
+        // Undo should be available
+        Assert.True(sheet.Commands.CanUndo);
+
+        // Undo the command
+        sheet.Commands.Undo();
+
+        // Filter should be removed
+        Assert.Empty(sheet.Filters);
+
+        // Redo should be available
+        Assert.True(sheet.Commands.CanRedo);
+
+        // Redo the command
+        sheet.Commands.Redo();
+
+        // Filter should be added again
+        Assert.Single(sheet.Filters);
+    }
+
+    [Fact]
+    public void Should_HandleMultipleFilters()
+    {
+        // Initially no filters
+        Assert.Empty(sheet.Filters);
+
+        // Create multiple filters
+        var filter1 = new SheetFilter(
+            new EqualToCriterion { Column = 0, Value = "Test1" },
+            RangeRef.Parse("A1:A5")
+        );
+
+        var filter2 = new SheetFilter(
+            new EqualToCriterion { Column = 1, Value = "Test2" },
+            RangeRef.Parse("B1:B5")
+        );
+
+        // Execute commands through the undo/redo stack
+        var command1 = new FilterCommand(sheet, filter1);
+        var command2 = new FilterCommand(sheet, filter2);
+
+        sheet.Commands.Execute(command1);
+        sheet.Commands.Execute(command2);
+
+        // Both filters should be added
+        Assert.Equal(2, sheet.Filters.Count);
+        Assert.Contains(filter1, sheet.Filters);
+        Assert.Contains(filter2, sheet.Filters);
+
+        // Undo both commands
+        sheet.Commands.Undo(); // Undo filter2
+        sheet.Commands.Undo(); // Undo filter1
+
+        // No filters should remain
+        Assert.Empty(sheet.Filters);
+
+        // Redo both commands
+        sheet.Commands.Redo(); // Redo filter1
+        sheet.Commands.Redo(); // Redo filter2
+
+        // Both filters should be back
+        Assert.Equal(2, sheet.Filters.Count);
+        Assert.Contains(filter1, sheet.Filters);
+        Assert.Contains(filter2, sheet.Filters);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/FindFunctionTests.cs

@@ -0,0 +1,51 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class FindFunctionTests
+{
+    [Fact]
+    public void Find_CaseSensitive_MatchesUppercase()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Miriam McGovern";
+        sheet.Cells["B1"].Formula = "=FIND(\"M\",A2)";
+        Assert.Equal(1d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Find_CaseSensitive_MatchesLowercase()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Miriam McGovern";
+        sheet.Cells["B1"].Formula = "=FIND(\"m\",A2)";
+        Assert.Equal(6d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Find_WithStartNum()
+    {
+        var sheet = new Sheet(10, 30);
+        sheet.Cells["A1"].Value = "AYF0093.YoungMensApparel";
+        sheet.Cells["B1"].Formula = "=FIND(\"Y\",A1,8)";
+        Assert.Equal(9d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Find_EmptyFindText_ReturnsStart()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "abc";
+        sheet.Cells["B1"].Formula = "=FIND(\"\",A1,2)";
+        Assert.Equal(2d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Find_NotFound_ReturnsValue()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "abc";
+        sheet.Cells["B1"].Formula = "=FIND(\"z\",A1)";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/FormulaEvaluatorTests.cs

@@ -0,0 +1,298 @@
+using System;
+using System.Linq.Expressions;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class FormulaEvaluationTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateFormulaAfterSettingIt()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Formula = "=A1+1";
+
+        Assert.Equal(2d, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateFormulaAfterSettingValue()
+    {
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Value = 1;
+
+        Assert.Equal(2d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldNotEvaluateFormulaIfEditing()
+    {
+        sheet.BeginUpdate();
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Value = 1;
+
+        Assert.Null(sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateFormulaAfterEndingEdit()
+    {
+        sheet.BeginUpdate();
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Value = 1;
+        sheet.EndUpdate();
+
+        Assert.Equal(2d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSetCellValueToErrorValueIfStringIsUsedInBinaryOperation()
+    {
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Value = "test";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSetCellValueToErrorNameIfInvalidFunctionIsUsedInFormula()
+    {
+        sheet.Cells["A1"].Formula = "=INVALID_FUNCTION()";
+        sheet.Cells["A2"].Value = "test";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Theory]
+    [InlineData("=SUM(")]
+    [InlineData("=SUM(A2,")]
+    [InlineData("=SUM(A2:A2")]
+    public void ShouldSetCellValueToErrorNameIfIncompleteFunctionIsUsedInFormula(string formula)
+    {
+        sheet.Cells["A1"].Formula = formula;
+        sheet.Cells["A2"].Value = "test";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSetCellValueToEqualsIfOnlyEqualsIsSetAsFormula()
+    {
+        sheet.Cells["A1"].SetValue("=");
+
+        Assert.Equal("=", sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateFormulaWhenDependencyIsChanged()
+    {
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Formula = "=A3+1";
+        sheet.Cells["A3"].Value = 1;
+
+        Assert.Equal(3d, sheet.Cells["A1"].Value);
+        Assert.Equal(2d, sheet.Cells["A2"].Value);
+        Assert.Equal(1d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateFormulaWhenDependencyIsChangedAndEndEditIsCalled()
+    {
+        sheet.BeginUpdate();
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Formula = "=A3+1";
+        sheet.Cells["A3"].Value = 1;
+        sheet.EndUpdate();
+
+        Assert.Equal(3d, sheet.Cells["A1"].Value);
+        Assert.Equal(2d, sheet.Cells["A2"].Value);
+        Assert.Equal(1d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldTreatEmptyValueAsZeroInFormula()
+    {
+        sheet.Cells["A1"].Formula = "=A2+1";
+
+        Assert.Equal(1d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldHandleSelfReferencingFormulas()
+    {
+        sheet.Cells["A1"].Formula = "=A1+1";
+
+        // Setting a value should not cause infinite recursion
+        sheet.Cells["A1"].Value = 1;
+
+        // The value should be stable and not cause infinite recursion
+        Assert.NotNull(sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSetDiv0ErrorWhenDividingByZero()
+    {
+        sheet.Cells["A1"].Formula = "=A2/A3";
+        sheet.Cells["A2"].Value = 1;
+        sheet.Cells["A3"].Value = 0;
+
+        Assert.Equal(CellError.Div0, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSetErrorToCircularWhenCellFormulasReferenceEachOther()
+    {
+        sheet.Cells["A1"].Formula = "=A2+1";
+        sheet.Cells["A2"].Formula = "=A1+1";
+
+        // The value should be an error
+        Assert.Equal(CellError.Circular, sheet.Cells["A1"].Value);
+        Assert.Equal(CellError.Circular, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNameErrorForUnknownFunction()
+    {
+        sheet.Cells["A1"].Formula = "=UNKNOWN()";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=A6";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=SUM(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenCountRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=COUNT(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCreateRefErrorWhenCountaRangeOutOfBounds()
+    {
+        sheet.Cells["A1"].Formula = "=COUNTA(A2:A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithDecimalValues()
+    {
+        sheet.Cells["A1"].Value = 0.5m;
+        sheet.Cells["A2"].Formula = "=IF(A1,\"True\",\"False\")";
+
+        Assert.Equal("True", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNameErrorForUnknownFunctionUppercase()
+    {
+        sheet.Cells["A1"].Formula = "=UNKNOWNFUNCTION(1,2,3)";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNameErrorForUnknownFunctionWithMixedCase()
+    {
+        sheet.Cells["A1"].Formula = "=UnknownFunction(1,2,3)";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNameErrorForUnknownFunctionWithLowercase()
+    {
+        sheet.Cells["A1"].Formula = "=unknownfunction(1,2,3)";
+
+        Assert.Equal(CellError.Name, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateAndFunctionInIfStatementWithFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 150;
+        sheet.Cells["A3"].Formula = "=IF(AND(A1>1,A2<100),A1,\"Out of range\")";
+
+        Assert.Equal("Out of range", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithProvidedExample3()
+    {
+        sheet.Cells["A2"].Value = 75;
+        sheet.Cells["A3"].Formula = "=IF(OR(A2<0,A2>50),A2,\"The value is out of range\")";
+
+        Assert.Equal(75d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithOrFunction()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=NOT(OR(A1,A2))";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    // IFERROR function tests are in IfErrorFunctionTests.cs
+
+    [Fact]
+    public void ShouldEvaluateSimpleDivisionByZero()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Formula = "=A1/0";
+
+        Assert.Equal(CellError.Div0, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void Evaluator_ShouldResolveCrossSheetCellReference()
+    {
+        var wb = new Workbook();
+        var s1 = wb.AddSheet("Sheet1", 5, 5);
+        var s2 = wb.AddSheet("Sheet2", 5, 5);
+
+        s2.Cells[0, 2].Value = 42; // C1 on Sheet2
+
+        s1.Cells[0, 0].Formula = "=Sheet2!C1"; // A1 on Sheet1 refers to Sheet2!C1
+
+        Assert.Equal(42d, s1.Cells[0, 0].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Evaluator_ShouldResolveCrossSheetRangeInFunction()
+    {
+        var wb = new Workbook();
+        var s1 = wb.AddSheet("Sheet1", 5, 5);
+        var s2 = wb.AddSheet("Sheet2", 5, 5);
+
+        s2.Cells[0, 0].Value = 1; // A1
+        s2.Cells[0, 1].Value = 2; // B1
+        s2.Cells[1, 0].Value = 3; // A2
+        s2.Cells[1, 1].Value = 4; // B2
+
+        s1.Cells[0, 0].Formula = "=SUM(Sheet2!A1:Sheet2!B2)";
+
+        Assert.Equal(10d, s1.Cells[0, 0].Data.GetValueOrDefault<double>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/FormulaLexerTests.cs

@@ -0,0 +1,143 @@
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+using System;
+using Xunit;
+
+public class FormulaLexerTests
+{
+    [Fact]
+    public void FormulaLexer_ShouldParseCellIdentifier()
+    {
+        var tokens = FormulaLexer.Scan("=A1");
+        Assert.Equal(FormulaTokenType.Equals, tokens[0].Type);
+        Assert.Equal(0, tokens[0].Start);
+        Assert.Equal(1, tokens[0].End);
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[1].Type);
+        Assert.Equal("A1", tokens[1].Address.ToString());
+        Assert.Equal(1, tokens[1].Start);
+        Assert.Equal(3, tokens[1].End);
+    }
+
+    [Fact]
+    public void FormulaLexer_ShouldParseSimpleFormula()
+    {
+        var tokens = FormulaLexer.Scan("=A1+b2");
+        Assert.Equal(FormulaTokenType.Equals, tokens[0].Type);
+        Assert.Equal(0, tokens[0].Start);
+        Assert.Equal(1, tokens[0].End);
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[1].Type);
+        Assert.Equal("A1", tokens[1].Value);
+        Assert.Equal(1, tokens[1].Start);
+        Assert.Equal(3, tokens[1].End);
+        Assert.Equal(FormulaTokenType.Plus, tokens[2].Type);
+        Assert.Equal(3, tokens[2].Start);
+        Assert.Equal(4, tokens[2].End);
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[3].Type);
+        Assert.Equal("b2", tokens[3].Value);
+        Assert.Equal(4, tokens[3].Start);
+        Assert.Equal(6, tokens[3].End);
+    }
+
+    [Fact]
+    public void FormulaLexer_ShouldPreserveWhitespaceAsTrivia()
+    {
+        var tokens = FormulaLexer.Scan("= A1 + b2 ");
+
+        // Check that whitespace is preserved as trivia
+        Assert.Equal(FormulaTokenType.Equals, tokens[0].Type);
+        Assert.Empty(tokens[0].LeadingTrivia);
+        Assert.Single(tokens[0].TrailingTrivia);
+        Assert.Equal(FormulaTokenTriviaKind.Whitespace, tokens[0].TrailingTrivia[0].Kind);
+        Assert.Equal(" ", tokens[0].TrailingTrivia[0].Text);
+        Assert.Equal(0, tokens[0].Start);
+        Assert.Equal(2, tokens[0].End);
+
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[1].Type);
+        Assert.Empty(tokens[1].LeadingTrivia);
+        Assert.Single(tokens[1].TrailingTrivia);
+        Assert.Equal(FormulaTokenTriviaKind.Whitespace, tokens[1].TrailingTrivia[0].Kind);
+        Assert.Equal(" ", tokens[1].TrailingTrivia[0].Text);
+        Assert.Equal(2, tokens[1].Start);
+        Assert.Equal(5, tokens[1].End);
+
+        Assert.Equal(FormulaTokenType.Plus, tokens[2].Type);
+        Assert.Empty(tokens[2].LeadingTrivia);
+        Assert.Single(tokens[2].TrailingTrivia);
+        Assert.Equal(FormulaTokenTriviaKind.Whitespace, tokens[2].TrailingTrivia[0].Kind);
+        Assert.Equal(" ", tokens[2].TrailingTrivia[0].Text);
+        Assert.Equal(5, tokens[2].Start);
+        Assert.Equal(7, tokens[2].End);
+
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[3].Type);
+        Assert.Empty(tokens[3].LeadingTrivia);
+        Assert.Single(tokens[3].TrailingTrivia);
+        Assert.Equal(FormulaTokenTriviaKind.Whitespace, tokens[3].TrailingTrivia[0].Kind);
+        Assert.Equal(" ", tokens[3].TrailingTrivia[0].Text);
+        Assert.Equal(7, tokens[3].Start);
+        Assert.Equal(10, tokens[3].End);
+    }
+
+    [Fact]
+    public void FormulaLexer_ShouldPreserveMultipleWhitespaceAsTrivia()
+    {
+        var tokens = FormulaLexer.Scan("=  A1  +  b2  ");
+
+        // Check that multiple whitespace characters are preserved
+        Assert.Equal(FormulaTokenType.Equals, tokens[0].Type);
+        Assert.Single(tokens[0].TrailingTrivia);
+        Assert.Equal("  ", tokens[0].TrailingTrivia[0].Text);
+        Assert.Equal(0, tokens[0].Start);
+        Assert.Equal(3, tokens[0].End);
+
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[1].Type);
+        Assert.Single(tokens[1].TrailingTrivia);
+        Assert.Equal("  ", tokens[1].TrailingTrivia[0].Text);
+        Assert.Equal(3, tokens[1].Start);
+        Assert.Equal(7, tokens[1].End);
+
+        Assert.Equal(FormulaTokenType.Plus, tokens[2].Type);
+        Assert.Single(tokens[2].TrailingTrivia);
+        Assert.Equal("  ", tokens[2].TrailingTrivia[0].Text);
+        Assert.Equal(7, tokens[2].Start);
+        Assert.Equal(10, tokens[2].End);
+
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[3].Type);
+        Assert.Single(tokens[3].TrailingTrivia);
+        Assert.Equal("  ", tokens[3].TrailingTrivia[0].Text);
+        Assert.Equal(10, tokens[3].Start);
+        Assert.Equal(14, tokens[3].End);
+    }
+
+    [Fact]
+    public void FormulaLexer_ShouldPreserveLineEndingsAsTrivia()
+    {
+        var tokens = FormulaLexer.Scan("=A1\n+b2");
+
+        // Check that line endings are preserved as trivia
+        Assert.Equal(FormulaTokenType.Equals, tokens[0].Type);
+        Assert.Empty(tokens[0].LeadingTrivia);
+        Assert.Empty(tokens[0].TrailingTrivia);
+        Assert.Equal(0, tokens[0].Start);
+        Assert.Equal(1, tokens[0].End);
+
+        Assert.Equal(FormulaTokenType.CellIdentifier, tokens[1].Type);
+        Assert.Empty(tokens[1].LeadingTrivia);
+        Assert.Single(tokens[1].TrailingTrivia);
+        Assert.Equal(FormulaTokenTriviaKind.EndOfLine, tokens[1].TrailingTrivia[0].Kind);
+        Assert.Equal("\n", tokens[1].TrailingTrivia[0].Text);
+        Assert.Equal(1, tokens[1].Start);
+        Assert.Equal(4, tokens[1].End);
+
+        Assert.Equal(FormulaTokenType.Plus, tokens[2].Type);
+        Assert.Empty(tokens[2].LeadingTrivia);
+        Assert.Empty(tokens[2].TrailingTrivia);
+        Assert.Equal(4, tokens[2].Start);
+        Assert.Equal(5, tokens[2].End);
+
+        Assert.Equal(expected: FormulaTokenType.CellIdentifier, tokens[3].Type);
+        Assert.Empty(tokens[3].LeadingTrivia);
+        Assert.Empty(tokens[3].TrailingTrivia);
+        Assert.Equal(5, tokens[3].Start);
+        Assert.Equal(7, tokens[3].End);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/FormulaParserTests.cs

@@ -0,0 +1,644 @@
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+using System;
+using Xunit;
+
+public class FormulaParserTests
+{
+    [Fact]
+    public void FormulaParser_ShouldRequireEqualsAtStart()
+    {
+        var formula = "A1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.Contains("Unexpected token", syntaxTree.Errors[0]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseNumberLiteral()
+    {
+        var formula = "=123";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<NumberLiteralSyntaxNode>(syntaxTree.Root);
+        var numberNode = (NumberLiteralSyntaxNode)syntaxTree.Root;
+        Assert.Equal(123, numberNode.Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseAdditionOfTwoNumberLiterals()
+    {
+        var formula = "=123+456";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<BinaryExpressionSyntaxNode>(syntaxTree.Root);
+        var binaryNode = (BinaryExpressionSyntaxNode)syntaxTree.Root;
+        Assert.Equal(BinaryOperator.Plus, binaryNode.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)binaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseAdditionOfMultipleNumberLiterals()
+    {
+        var formula = "=123+456+789";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Plus, binaryNode.Operator);
+        Assert.IsType<BinaryExpressionSyntaxNode>(binaryNode.Left);
+        var leftBinaryNode = (BinaryExpressionSyntaxNode)binaryNode.Left;
+        Assert.Equal(BinaryOperator.Plus, leftBinaryNode.Operator);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)leftBinaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)leftBinaryNode.Right).Token.IntValue);
+        Assert.Equal(789, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseSubtractionOfTwoNumberLiterals()
+    {
+        var formula = "=123-456";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Minus, binaryNode.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)binaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseUnaryNegativeNumber()
+    {
+        var formula = "=-123";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<UnaryExpressionSyntaxNode>(syntaxTree.Root);
+        var unary = (UnaryExpressionSyntaxNode)syntaxTree.Root;
+        Assert.Equal(UnaryOperator.Negate, unary.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(unary.Operand);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)unary.Operand).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseUnaryPlusNumber()
+    {
+        var formula = "=+123";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<UnaryExpressionSyntaxNode>(syntaxTree.Root);
+        var unary = (UnaryExpressionSyntaxNode)syntaxTree.Root;
+        Assert.Equal(UnaryOperator.Plus, unary.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(unary.Operand);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)unary.Operand).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseUnaryPlusInFunctionArgument()
+    {
+        var formula = "=LEFT(A1,+1)";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<FunctionSyntaxNode>(syntaxTree.Root);
+        var fn = (FunctionSyntaxNode)syntaxTree.Root;
+        Assert.Equal("LEFT", fn.Name);
+        Assert.IsType<UnaryExpressionSyntaxNode>(fn.Arguments[1]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseMultipleUnaryOperators()
+    {
+        var formula = "=-+-+3";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        // Expect nested unary nodes: - ( + ( - ( + 3 ) ) )
+        var node = syntaxTree.Root;
+        Assert.IsType<UnaryExpressionSyntaxNode>(node);
+        var u1 = (UnaryExpressionSyntaxNode)node; // '-'
+        Assert.Equal(UnaryOperator.Negate, u1.Operator);
+        Assert.IsType<UnaryExpressionSyntaxNode>(u1.Operand);
+        var u2 = (UnaryExpressionSyntaxNode)u1.Operand; // '+'
+        Assert.Equal(UnaryOperator.Plus, u2.Operator);
+        Assert.IsType<UnaryExpressionSyntaxNode>(u2.Operand);
+        var u3 = (UnaryExpressionSyntaxNode)u2.Operand; // '-'
+        Assert.Equal(UnaryOperator.Negate, u3.Operator);
+        Assert.IsType<UnaryExpressionSyntaxNode>(u3.Operand);
+        var u4 = (UnaryExpressionSyntaxNode)u3.Operand; // '+'
+        Assert.Equal(UnaryOperator.Plus, u4.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(u4.Operand);
+        Assert.Equal(3, ((NumberLiteralSyntaxNode)u4.Operand).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseUnaryNegativeInFunctionArgument()
+    {
+        var formula = "=LEFT(A1,-1)";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<FunctionSyntaxNode>(syntaxTree.Root);
+        var fn = (FunctionSyntaxNode)syntaxTree.Root;
+        Assert.Equal("LEFT", fn.Name);
+        Assert.Equal(2, fn.Arguments.Count);
+        Assert.IsType<CellSyntaxNode>(fn.Arguments[0]);
+        Assert.IsType<UnaryExpressionSyntaxNode>(fn.Arguments[1]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseSubtractionOfMultipleNumberLiterals()
+    {
+        var formula = "=123-456-789";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Minus, binaryNode.Operator);
+        Assert.IsType<BinaryExpressionSyntaxNode>(binaryNode.Left);
+        var leftBinaryNode = (BinaryExpressionSyntaxNode)binaryNode.Left;
+        Assert.Equal(BinaryOperator.Minus, leftBinaryNode.Operator);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)leftBinaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)leftBinaryNode.Right).Token.IntValue);
+        Assert.Equal(789, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseMultiplicationOfTwoNumberLiterals()
+    {
+        var formula = "=123*456";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Multiply, binaryNode.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)binaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParse_MultiplicationPrecedence()
+    {
+        var formula = "=123+456*789";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Plus, binaryNode.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+        Assert.IsType<BinaryExpressionSyntaxNode>(binaryNode.Right);
+        var rightBinaryNode = (BinaryExpressionSyntaxNode)binaryNode.Right;
+        Assert.Equal(BinaryOperator.Multiply, rightBinaryNode.Operator);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)rightBinaryNode.Left).Token.IntValue);
+        Assert.Equal(789, ((NumberLiteralSyntaxNode)rightBinaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseDivisionOfTwoNumberLiterals()
+    {
+        var formula = "=123/456";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Divide, binaryNode.Operator);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)binaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseParentheses()
+    {
+        var formula = "=(123+456)*789";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Multiply, binaryNode.Operator);
+        Assert.IsType<BinaryExpressionSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        var leftBinaryNode = (BinaryExpressionSyntaxNode)binaryNode.Left;
+        Assert.Equal(789, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+        Assert.Equal(BinaryOperator.Plus, leftBinaryNode.Operator);
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)leftBinaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)leftBinaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseNestedParentheses()
+    {
+        var formula = "=((123+456)*789)/101112";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+        var binaryNode = (BinaryExpressionSyntaxNode)node;
+        Assert.Equal(BinaryOperator.Divide, binaryNode.Operator);
+        Assert.IsType<BinaryExpressionSyntaxNode>(binaryNode.Left);
+        Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Right);
+        var leftBinaryNode = (BinaryExpressionSyntaxNode)binaryNode.Left;
+        Assert.Equal(101112, ((NumberLiteralSyntaxNode)binaryNode.Right).Token.IntValue);
+        Assert.Equal(BinaryOperator.Multiply, leftBinaryNode.Operator);
+        Assert.IsType<BinaryExpressionSyntaxNode>(leftBinaryNode.Left);
+        var leftLeftBinaryNode = (BinaryExpressionSyntaxNode)leftBinaryNode.Left;
+        Assert.Equal(123, ((NumberLiteralSyntaxNode)leftLeftBinaryNode.Left).Token.IntValue);
+        Assert.Equal(456, ((NumberLiteralSyntaxNode)leftLeftBinaryNode.Right).Token.IntValue);
+        Assert.Equal(789, ((NumberLiteralSyntaxNode)leftBinaryNode.Right).Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseCellIndentifer()
+    {
+        var formula = "=A1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<CellSyntaxNode>(node);
+        var cellIdentifierNode = (CellSyntaxNode)node;
+        Assert.Equal("A1", cellIdentifierNode.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseSheetQualifiedCellIdentifier()
+    {
+        var formula = "=Sheet2!C1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<CellSyntaxNode>(node);
+        var cellIdentifierNode = (CellSyntaxNode)node;
+        Assert.Equal("C1", cellIdentifierNode.Token.Address.ToString());
+        Assert.Equal("Sheet2", cellIdentifierNode.Token.Address.Sheet);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseFunction()
+    {
+        var formula = "=SUM(A1,1)";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<FunctionSyntaxNode>(node);
+        var functionNode = (FunctionSyntaxNode)node;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.Equal(2, functionNode.Arguments.Count);
+        Assert.IsType<CellSyntaxNode>(functionNode.Arguments[0]);
+        Assert.IsType<NumberLiteralSyntaxNode>(functionNode.Arguments[1]);
+
+        var cellIdentifierNode = (CellSyntaxNode)functionNode.Arguments[0];
+        Assert.Equal("A1", cellIdentifierNode.Token.Address.ToString());
+        var numberLiteralNode = (NumberLiteralSyntaxNode)functionNode.Arguments[1];
+        Assert.Equal(1, numberLiteralNode.Token.IntValue);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseNestedFunctions()
+    {
+        var formula = "=SUM(A1,MAX(B1,C1))";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<FunctionSyntaxNode>(node);
+        var functionNode = (FunctionSyntaxNode)node;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.Equal(2, functionNode.Arguments.Count);
+        Assert.IsType<CellSyntaxNode>(functionNode.Arguments[0]);
+        Assert.IsType<FunctionSyntaxNode>(functionNode.Arguments[1]);
+
+        var cellIdentifierNode = (CellSyntaxNode)functionNode.Arguments[0];
+        Assert.Equal("A1", cellIdentifierNode.Token.Address.ToString());
+
+        var nestedFunctionNode = (FunctionSyntaxNode)functionNode.Arguments[1];
+        Assert.Equal("MAX", nestedFunctionNode.Name);
+        Assert.Equal(2, nestedFunctionNode.Arguments.Count);
+        Assert.IsType<CellSyntaxNode>(nestedFunctionNode.Arguments[0]);
+        Assert.IsType<CellSyntaxNode>(nestedFunctionNode.Arguments[1]);
+
+        var firstCellIdentifierNode = (CellSyntaxNode)nestedFunctionNode.Arguments[0];
+        var secondCellIdentifierNode = (CellSyntaxNode)nestedFunctionNode.Arguments[1];
+        Assert.Equal("B1", firstCellIdentifierNode.Token.Address.ToString());
+        Assert.Equal("C1", secondCellIdentifierNode.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseFunctionWithNoArguments()
+    {
+        var formula = "=SUM()";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<FunctionSyntaxNode>(node);
+        var functionNode = (FunctionSyntaxNode)node;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.Empty(functionNode.Arguments);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseCellRange()
+    {
+        var formula = "=A1:A2";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("A2", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseSheetQualifiedRange()
+    {
+        var formula = "=Sheet2!A1:Sheet2!B2";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("B2", rangeNode.End.Token.Address.ToString());
+        Assert.Equal("Sheet2", rangeNode.Start.Token.Address.Sheet);
+        Assert.Equal("Sheet2", rangeNode.End.Token.Address.Sheet);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseCellRangeInFunction()
+    {
+        var formula = "=SUM(A1:A2)";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<FunctionSyntaxNode>(node);
+        var functionNode = (FunctionSyntaxNode)node;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.Single(functionNode.Arguments);
+        Assert.IsType<RangeSyntaxNode>(functionNode.Arguments[0]);
+        var rangeNode = (RangeSyntaxNode)functionNode.Arguments[0];
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("A2", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleInvalidRange()
+    {
+        var formula = "=A2:A1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A2", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("A1", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleSingleCellRange()
+    {
+        var formula = "=A1:A1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("A1", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleMultiColumnRange()
+    {
+        var formula = "=A1:B1";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("B1", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleMultiRowRange()
+    {
+        var formula = "=A1:A2";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("A2", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleMultiCellRange()
+    {
+        var formula = "=A1:B2";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.IsType<RangeSyntaxNode>(node);
+        var rangeNode = (RangeSyntaxNode)node;
+        Assert.Equal("A1", rangeNode.Start.Token.Address.ToString());
+        Assert.Equal("B2", rangeNode.End.Token.Address.ToString());
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldAddErrorOnInvalidFormula()
+    {
+        var formula = "A1"; // Missing equals sign
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.Contains("Unexpected token", syntaxTree.Errors[0]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldReturnPartialExpressionOnIncompleteExpression()
+    {
+        var formula = "=123+"; // Incomplete expression
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.IsType<BinaryExpressionSyntaxNode>(syntaxTree.Root);
+        if (syntaxTree.Root is BinaryExpressionSyntaxNode binaryNode)
+        {
+            Assert.Equal(BinaryOperator.Plus, binaryNode.Operator);
+            Assert.IsType<NumberLiteralSyntaxNode>(binaryNode.Left);
+            Assert.Equal(123, ((NumberLiteralSyntaxNode)binaryNode.Left).Token.IntValue);
+        }
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldAddErrorOnIncompleteExpression()
+    {
+        var formula = "=123+"; // Incomplete expression
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldReturnPartialFunctionOnMissingCloseParen()
+    {
+        var formula = "=SUM(A1"; // Missing closing parenthesis
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.IsType<FunctionSyntaxNode>(syntaxTree.Root);
+        var functionNode = (FunctionSyntaxNode)syntaxTree.Root;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.Single(functionNode.Arguments);
+        Assert.IsType<CellSyntaxNode>(functionNode.Arguments[0]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldAddErrorOnInvalidFunctionSyntax()
+    {
+        var formula = "=SUM(A1"; // Missing closing parenthesis
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseGroupedExpression()
+    {
+        var formula = "=(A1)"; // Parentheses without function name should parse as grouped expression
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors); // This should actually succeed as it's a valid grouped expression
+        Assert.IsType<CellSyntaxNode>(syntaxTree.Root);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldReturnPartialRangeOnIncompleteRange()
+    {
+        var formula = "=A1:"; // Incomplete range
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.NotNull(syntaxTree.Root);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldAddErrorOnInvalidRange()
+    {
+        var formula = "=A1:"; // Incomplete range
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleUnterminatedString()
+    {
+        var formula = "=\"hello"; // Unterminated string literal
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors); // Should succeed as lexer handles unterminated strings
+        Assert.IsType<StringLiteralSyntaxNode>(syntaxTree.Root);
+        var stringNode = (StringLiteralSyntaxNode)syntaxTree.Root;
+        Assert.Equal("hello", stringNode.Token.Value);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldHandleMissingOperand()
+    {
+        var formula = "=*5"; // Missing left operand
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors); // Should have errors
+        Assert.NotNull(syntaxTree.Root);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldReturnPartialExpressionOnUnbalancedParentheses()
+    {
+        var formula = "=(A1+B1"; // Missing closing parenthesis
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.IsType<BinaryExpressionSyntaxNode>(syntaxTree.Root); // Should return the binary expression inside
+        var binaryNode = (BinaryExpressionSyntaxNode)syntaxTree.Root;
+        Assert.Equal(BinaryOperator.Plus, binaryNode.Operator);
+        Assert.IsType<CellSyntaxNode>(binaryNode.Left);
+        Assert.IsType<CellSyntaxNode>(binaryNode.Right);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldAddErrorOnUnbalancedParentheses()
+    {
+        var formula = "=(A1+B1"; // Missing closing parenthesis
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldReturnPartialFunctionOnIncompleteArguments()
+    {
+        var formula = "=SUM(A1,"; // Incomplete function arguments
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+        Assert.IsType<FunctionSyntaxNode>(syntaxTree.Root);
+        var functionNode = (FunctionSyntaxNode)syntaxTree.Root;
+        Assert.Equal("SUM", functionNode.Name);
+        Assert.True(functionNode.Arguments.Count >= 1); // Should have at least the first argument
+        Assert.IsType<CellSyntaxNode>(functionNode.Arguments[0]);
+    }
+
+    [Fact]
+    public void FormulaParser_DefaultBehavior_ShouldStillWork()
+    {
+        // Test that default behavior still works as before
+        var formula = "=123+456";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var node = syntaxTree.Root;
+        Assert.NotNull(node);
+        Assert.IsType<BinaryExpressionSyntaxNode>(node);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseBooleanTrue()
+    {
+        var formula = "=TRUE";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<BooleanLiteralSyntaxNode>(syntaxTree.Root);
+        var boolNode = (BooleanLiteralSyntaxNode)syntaxTree.Root;
+        Assert.Equal("TRUE", boolNode.Token.Value);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseBooleanFalse_Lowercase()
+    {
+        var formula = "=false";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        Assert.IsType<BooleanLiteralSyntaxNode>(syntaxTree.Root);
+        var boolNode = (BooleanLiteralSyntaxNode)syntaxTree.Root;
+        Assert.Equal("false", boolNode.Token.Value);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParseBooleanInFunction()
+    {
+        var formula = "=TEXTJOIN(\", \", TRUE, A1:A2)";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.Empty(syntaxTree.Errors);
+        var fn = Assert.IsType<FunctionSyntaxNode>(syntaxTree.Root);
+        Assert.Equal("TEXTJOIN", fn.Name);
+        Assert.IsType<BooleanLiteralSyntaxNode>(fn.Arguments[1]);
+    }
+
+    [Fact]
+    public void FormulaParser_ShouldParse_Percent()
+    {
+        var formula = "=$%";
+        var syntaxTree = FormulaParser.Parse(formula);
+        Assert.NotEmpty(syntaxTree.Errors);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/FunctionRegistryTests.cs

@@ -0,0 +1,74 @@
+using System;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class FunctionRegistryTests
+{
+    private readonly FunctionStore functionRegistry = new();
+
+    [Theory]
+    [InlineData(2, -1)]
+    [InlineData(4, -1)]
+    [InlineData(5, 0)]
+    [InlineData(6, 0)]
+    [InlineData(7, 1)]
+    [InlineData(8, 1)]
+    [InlineData(9, 2)]
+    [InlineData(10, 2)]
+    public void Basic_Function_Provides_Correct_Arg_Index(int cursorPosition, int expectedArgIndex)
+    {
+        var func = "=SUM(1,2,3)";
+        var result = functionRegistry.CreateFunctionHint(func, cursorPosition);
+
+        Assert.NotNull(result);
+
+        Assert.Equal(expectedArgIndex, result.ArgumentIndex);
+        Assert.IsType<SumFunction>(result.Function);
+    }
+    [Theory]
+    [InlineData(5, 0, "=SUM(")]
+    [InlineData(4, -1, "=SUM(")]
+    public void Basic_Function_Provides_Correct_Arg_Index_With_IncompleteFormula(int cursorPosition, int expectedArgIndex, string formula)
+    {
+        var result = functionRegistry.CreateFunctionHint(formula, cursorPosition);
+
+        Assert.NotNull(result);
+
+        Assert.Equal(expectedArgIndex, result.ArgumentIndex);
+        Assert.IsType<SumFunction>(result.Function);
+    }
+
+    [Fact]
+    public void Position_Outside_Of_Formula_Returns_null()
+    {
+        var func = "=1 + SUM(1,2, 3) + 2";
+        var result = functionRegistry.CreateFunctionHint(func, 0);
+
+        Assert.Null(result);
+
+        result = functionRegistry.CreateFunctionHint(func, 5);
+        Assert.Null(result);
+
+        result = functionRegistry.CreateFunctionHint(func, 16);
+        Assert.Null(result);
+    }
+
+    [Theory]
+    [InlineData(5, 0, typeof(SumFunction))]
+    [InlineData(7, 1, typeof(SumFunction))]
+    [InlineData(8, -1, typeof(CountFunction))]
+    [InlineData(13, 0, typeof(CountFunction))]
+    [InlineData(15, 1, typeof(CountFunction))]
+    [InlineData(17, 1, typeof(SumFunction))]
+    public void Nested_Function_Produces_Correct_ArgIndex(int cursorPosition, int expectedArgIndex, Type expectedFunction)
+    {
+        var func = "=SUM(1,COUNT(1,2),3)";
+        var result = functionRegistry.CreateFunctionHint(func, cursorPosition);
+
+        Assert.NotNull(result);
+
+        Assert.Equal(expectedArgIndex, result.ArgumentIndex);
+        Assert.IsType(expectedFunction, result.Function);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/HorizontalLookupFunctionTests.cs

@@ -0,0 +1,59 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class HorizontalLookupFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldFindExactMatchInTwoRowRange()
+    {
+        sheet.Cells["A1"].Value = "Size";
+        sheet.Cells["B1"].Value = "Color";
+        sheet.Cells["A2"].Value = "M";
+        sheet.Cells["B2"].Value = "Blue";
+
+        sheet.Cells["C1"].Formula = "=HLOOKUP(\"Color\",A1:B2,2,0)";
+
+        Assert.Equal("Blue", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNAWhenNoExactMatch()
+    {
+        sheet.Cells["A1"].Value = "Size";
+        sheet.Cells["A2"].Value = "M";
+
+        sheet.Cells["B1"].Formula = "=HLOOKUP(\"Color\",A1:A2,2,0)";
+
+        Assert.Equal(CellError.NA, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldFindApproximateMatchInSortedTopRow()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["B1"].Value = 20;
+        sheet.Cells["C1"].Value = 30;
+
+        sheet.Cells["A2"].Value = "Low";
+        sheet.Cells["B2"].Value = "Medium";
+        sheet.Cells["C2"].Value = "High";
+
+        sheet.Cells["D1"].Formula = "=HLOOKUP(25,A1:C2,2,1)";
+
+        Assert.Equal("Medium", sheet.Cells["D1"].Value);
+    }
+
+    [Fact]
+    public void ShouldErrorWhenIndexOutOfRange()
+    {
+        sheet.Cells["A1"].Value = "X";
+        sheet.Cells["A2"].Value = 1;
+
+        sheet.Cells["B1"].Formula = "=HLOOKUP(\"X\",A1:A2,3,0)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["B1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/HourFunctionTests.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class HourFunctionTests
+{
+    [Fact]
+    public void Hour_FromFraction_Returns18()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Data = CellData.FromNumber(0.75); // 18:00
+        sheet.Cells["B2"].Formula = "=HOUR(A2)";
+        Assert.Equal(18, sheet.Cells["B2"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Hour_FromDateTimeValue_ReturnsHour()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Data = CellData.FromDate(new System.DateTime(2011, 7, 18, 7, 45, 0));
+        sheet.Cells["B3"].Formula = "=HOUR(A3)";
+        Assert.Equal(7, sheet.Cells["B3"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Hour_FromDateOnly_ReturnsZero()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A4"].Data = CellData.FromDate(new System.DateTime(2012, 4, 21));
+        sheet.Cells["B4"].Formula = "=HOUR(A4)";
+        Assert.Equal(0, sheet.Cells["B4"].Data.GetValueOrDefault<double>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/IfErrorFunctionTests.cs

@@ -0,0 +1,138 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class IfErrorFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithNoError()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, \"Error in calculation\")";
+
+        Assert.Equal(5d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithDivisionByZero()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, \"Error in calculation\")";
+
+        Assert.Equal("Error in calculation", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithReferenceError()
+    {
+        sheet.Cells["A1"].Formula = "=IFERROR(A6, \"Error in calculation\")";
+
+        Assert.Equal("Error in calculation", sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithEmptyStringForError()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, \"\")";
+
+        Assert.Equal("", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithNumericErrorValue()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, 0)";
+
+        Assert.Equal(0d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithEmptyCellAsErrorValue()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, A4)";
+
+        Assert.Equal("", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithEmptyCellAsValue()
+    {
+        sheet.Cells["A1"].Formula = "=IFERROR(A2, \"Empty cell\")";
+
+        Assert.Equal("", sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithStringValue()
+    {
+        sheet.Cells["A1"].Value = "Hello";
+        sheet.Cells["A2"].Formula = "=IFERROR(A1, \"Error\")";
+
+        Assert.Equal("Hello", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithBooleanValue()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Formula = "=IFERROR(A1, \"Error\")";
+
+        Assert.Equal(true, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForIfErrorTooFewArguments()
+    {
+        sheet.Cells["A1"].Formula = "=IFERROR(A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForIfErrorTooManyArguments()
+    {
+        sheet.Cells["A1"].Formula = "=IFERROR(A2, \"Error\", \"Extra\")";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithNestedFormula()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=IFERROR(A1/A2, IFERROR(A1/0, \"Nested Error\"))";
+
+        Assert.Equal("Nested Error", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithSumFunction()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Formula = "=IFERROR(SUM(A1:A2), \"Error\")";
+
+        Assert.Equal(30d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfErrorFunctionWithSumFunctionError()
+    {
+        sheet.Cells["A1"].Formula = "=IFERROR(SUM(A6:A8), \"Error\")";
+
+        Assert.Equal("Error", sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/IfFunctionTests.cs

@@ -0,0 +1,178 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class IfFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithTrueCondition()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,\"Yes\",\"No\")";
+
+        Assert.Equal("Yes", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 2;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,\"Yes\",\"No\")";
+
+        Assert.Equal("No", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithNumericComparison()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 5;
+        sheet.Cells["A3"].Formula = "=IF(A1>A2,\"Over Budget\",\"Within Budget\")";
+
+        Assert.Equal("Over Budget", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithNumericResult()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 5;
+        sheet.Cells["A3"].Formula = "=IF(A1>A2,A1-A2,0)";
+
+        Assert.Equal(5d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithTwoArguments()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,\"True\")";
+
+        Assert.Equal("True", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithTwoArgumentsFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,\"True\")";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithZeroAsFalse()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Formula = "=IF(A1,\"True\",\"False\")";
+
+        Assert.Equal("False", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithNonZeroAsTrue()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Formula = "=IF(A1,\"True\",\"False\")";
+
+        Assert.Equal("True", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateToErrorIfFunctionWithStringCondition()
+    {
+        sheet.Cells["A1"].Value = "test";
+        sheet.Cells["A2"].Formula = "=IF(A1,\"Not Empty\",\"Empty\")";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithEmptyStringCondition()
+    {
+        sheet.Cells["A2"].Formula = "=IF(A1,\"Not Empty\",\"Empty\")";
+
+        Assert.Equal("Empty", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithNullCondition()
+    {
+        sheet.Cells["A2"].Formula = "=IF(A1,\"Not Empty\",\"Empty\")";
+
+        Assert.Equal("Empty", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForTooFewArguments()
+    {
+        sheet.Cells["A1"].Formula = "=IF(A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForTooManyArguments()
+    {
+        sheet.Cells["A1"].Formula = "=IF(A2,\"True\",\"False\",\"Extra\")";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldPropagateErrorFromCondition()
+    {
+        sheet.Cells["A1"].Formula = "=IF(A6,\"True\",\"False\")";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldPropagateErrorFromTrueValue()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,A6,\"False\")";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldPropagateErrorFromFalseValue()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Formula = "=IF(A1=1,\"True\",A6)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNestedIfFunction()
+    {
+        sheet.Cells["A1"].Value = 85;
+        sheet.Cells["A2"].Formula = "=IF(A1>=90,\"A\",IF(A1>=80,\"B\",IF(A1>=70,\"C\",\"F\")))";
+
+        Assert.Equal("B", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithBooleanValues()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Formula = "=IF(A1,\"True\",\"False\")";
+
+        Assert.Equal("True", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateIfFunctionWithDecimalValues()
+    {
+        sheet.Cells["A1"].Value = 0.5m;
+        sheet.Cells["A2"].Formula = "=IF(A1,\"True\",\"False\")";
+
+        Assert.Equal("True", sheet.Cells["A2"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/IndexFunctionTests.cs

@@ -0,0 +1,100 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class IndexFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    void Seed()
+    {
+        sheet.Cells["A2"].Value = "Apples";
+        sheet.Cells["B2"].Value = "Lemons";
+        sheet.Cells["A3"].Value = "Bananas";
+        sheet.Cells["B3"].Value = "Pears";
+    }
+
+    [Fact]
+    public void ShouldReturnIntersectionValue()
+    {
+        Seed();
+
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,2,2)";
+        Assert.Equal("Pears", sheet.Cells["C1"].Value);
+
+        sheet.Cells["C2"].Formula = "=INDEX(A2:B3,2,1)";
+        Assert.Equal("Bananas", sheet.Cells["C2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnRefErrorIfOutOfRange()
+    {
+        // numeric values just to ensure range exists
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["B1"].Value = 2;
+        sheet.Cells["A2"].Value = 3;
+        sheet.Cells["B2"].Value = 4;
+
+        sheet.Cells["C1"].Formula = "=INDEX(A1:B2,3,1)"; // row 3 out of 2 rows
+        Assert.Equal(CellError.Ref, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldDefaultColumnToFirstWhenOmitted()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,2)"; // column omitted -> first column
+        Assert.Equal("Bananas", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldUseAreaOneWhenSpecified()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,2,2,1)";
+        Assert.Equal("Pears", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorWhenAreaGreaterThanOne()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,1,1,2)";
+        Assert.Equal(CellError.Value, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnFirstOfEntireColumnWhenRowIsZero()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,0,2)";
+        Assert.Equal("Lemons", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnFirstOfEntireRowWhenColumnIsZero()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,2,0)";
+        Assert.Equal("Bananas", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorWhenBothRowAndColumnOmitted()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3)";
+        Assert.Equal(CellError.Value, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnRefErrorOnNegativeIndices()
+    {
+        Seed();
+        sheet.Cells["C1"].Formula = "=INDEX(A2:B3,0-1,1)"; // -1
+        Assert.Equal(CellError.Ref, sheet.Cells["C1"].Value);
+
+        sheet.Cells["C2"].Formula = "=INDEX(A2:B3,1,0-1)"; // -1
+        Assert.Equal(CellError.Ref, sheet.Cells["C2"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/InsertRowColumnTests.cs

@@ -0,0 +1,64 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class InsertRowColumnTests
+{
+    [Fact]
+    public void InsertColumn_ShiftsReferencesAndValues()
+    {
+        var sheet = new Sheet(5, 5);
+
+        sheet.Cells[1, 0].Value = 1;   // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+        Assert.Equal(11d, sheet.Cells[1, 1].Value);
+
+        // Insert a column before A (index 0)
+        sheet.InsertColumn(0, 1);
+
+        // Values shift right
+        Assert.Equal(1d, sheet.Cells[1, 1].Value); // A2 moved to B2
+
+        // Formula shifts position and updates referenced address
+        Assert.Equal("=B2+10", sheet.Cells[1, 2].Formula); // original B2 moved to C2
+        Assert.Equal(11d, sheet.Cells[1, 2].Value);
+    }
+
+    [Fact]
+    public void InsertRow_ShiftsReferencesAndValues()
+    {
+        var sheet = new Sheet(5, 5);
+
+        sheet.Cells[1, 0].Value = 1;   // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+        Assert.Equal(11d, sheet.Cells[1, 1].Value);
+
+        // Insert a row before row 2 (index 1)
+        sheet.InsertRow(1, 1);
+
+        // Values shift down
+        Assert.Equal(1d, sheet.Cells[2, 0].Value); // A2 moved to A3
+
+        // Formula shifts position and updates referenced address
+        Assert.Equal("=A3+10", sheet.Cells[2, 1].Formula); // original B2 moved to B3
+        Assert.Equal(11d, sheet.Cells[2, 1].Value);
+    }
+
+    [Fact]
+    public void InsertRow_IncreasesRowCount()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.InsertRow(2, 2);
+        Assert.Equal(7, sheet.RowCount);
+    }
+
+    [Fact]
+    public void InsertColumn_IncreasesColumnCount()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.InsertColumn(3, 3);
+        Assert.Equal(8, sheet.ColumnCount);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/IntFunctionTests.cs

@@ -0,0 +1,30 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class IntFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldRoundDownPositive()
+    {
+        sheet.Cells["A1"].Formula = "=INT(8.9)";
+        Assert.Equal(8d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundDownNegative()
+    {
+        sheet.Cells["A1"].Formula = "=INT(0-8.9)";
+        Assert.Equal(-9d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnDecimalPart()
+    {
+        sheet.Cells["A2"].Value = 19.5;
+        sheet.Cells["A1"].Formula = "=A2-INT(A2)";
+        Assert.Equal(0.5, sheet.Cells["A1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/LargeFunctionTests.cs

@@ -0,0 +1,74 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class LargeFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldReturnKthLargestAcrossRange()
+    {
+        // Populate A2:B6 as in example
+        sheet.Cells["A2"].Value = 3;
+        sheet.Cells["A3"].Value = 4;
+        sheet.Cells["A4"].Value = 5;
+        sheet.Cells["A5"].Value = 2;
+        sheet.Cells["A6"].Value = 3;
+
+        sheet.Cells["B2"].Value = 4;
+        sheet.Cells["B3"].Value = 5;
+        sheet.Cells["B4"].Value = 6;
+        sheet.Cells["B5"].Value = 4;
+        sheet.Cells["B6"].Value = 7;
+
+        sheet.Cells["C1"].Formula = "=LARGE(A2:B6,3)";
+
+        Assert.Equal(5d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturn7thLargestAsFour()
+    {
+        sheet.Cells["A2"].Value = 3;
+        sheet.Cells["A3"].Value = 4;
+        sheet.Cells["A4"].Value = 5;
+        sheet.Cells["A5"].Value = 2;
+        sheet.Cells["A6"].Value = 3;
+
+        sheet.Cells["B2"].Value = 4;
+        sheet.Cells["B3"].Value = 5;
+        sheet.Cells["B4"].Value = 6;
+        sheet.Cells["B5"].Value = 4;
+        sheet.Cells["B6"].Value = 7;
+
+        sheet.Cells["C1"].Formula = "=LARGE(A2:B6,7)";
+
+        Assert.Equal(4d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNumErrorForInvalidK()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["A3"].Value = 3;
+
+        sheet.Cells["B1"].Formula = "=LARGE(A1:A3,0)";
+        Assert.Equal(CellError.Num, sheet.Cells["B1"].Value);
+
+        sheet.Cells["B2"].Formula = "=LARGE(A1:A3,5)";
+        Assert.Equal(CellError.Num, sheet.Cells["B2"].Value);
+    }
+
+    [Fact]
+    public void ShouldIgnoreNonNumericCellsInArray()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "x"; // ignored
+        sheet.Cells["A3"].Value = 7;
+
+        sheet.Cells["B1"].Formula = "=LARGE(A1:A3,2)";
+        Assert.Equal(7d, sheet.Cells["B1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/LeftFunctionTests.cs

@@ -0,0 +1,42 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class LeftFunctionTests
+{
+    [Fact]
+    public void Left_WithCount_ReturnsPrefix()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Sale Price";
+        sheet.Cells["B1"].Formula = "=LEFT(A2,4)";
+        Assert.Equal("Sale", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Left_OmittedCount_DefaultsToOne()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "Sweden";
+        sheet.Cells["B1"].Formula = "=LEFT(A3)";
+        Assert.Equal("S", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Left_CountExceedsLength_ReturnsWhole()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Hi";
+        sheet.Cells["B1"].Formula = "=LEFT(A1,5)";
+        Assert.Equal("Hi", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Left_NegativeCount_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Test";
+        sheet.Cells["B1"].Formula = "=LEFT(A1,-1)";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/LenFunctionTests.cs

@@ -0,0 +1,59 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class LenFunctionTests
+{
+    [Fact]
+    public void Len_String_ReturnsLength()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Phoenix, AZ"; // 11 characters
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(11d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Len_BooleanCellTrue_ReturnsFour()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(4d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Len_BooleanCellFalse_ReturnsFive()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(5d, sheet.Cells["B1"].Data.Value);
+    }
+    [Fact]
+    public void Len_Empty_ReturnsZero()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = null; // empty
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(0d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Len_String_WithSpaces_CountsSpaces()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "     One   "; // 11 characters including spaces
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(11d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Len_Number_TreatsAsTextLength()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = 123.45; // "123.45" length 6
+        sheet.Cells["B1"].Formula = "=LEN(A1)";
+        Assert.Equal(6d, sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/LowerFunctionTests.cs

@@ -0,0 +1,24 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class LowerFunctionTests
+{
+    [Fact]
+    public void Lower_ConvertsToLowercase()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "E. E. Cummings";
+        sheet.Cells["B1"].Formula = "=LOWER(A2)";
+        Assert.Equal("e. e. cummings", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Lower_IgnoresNonLetters()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "Apt. 2B";
+        sheet.Cells["B1"].Formula = "=LOWER(A3)";
+        Assert.Equal("apt. 2b", sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/MaxAllFunctionTests.cs

@@ -0,0 +1,64 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MaxAllFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldEvaluateLogicalValuesInRange()
+    {
+        sheet.Cells["A1"].Value = true;   // 1
+        sheet.Cells["A2"].Value = false;  // 0
+        sheet.Cells["A3"].Value = 5;      // 5
+
+        sheet.Cells["B1"].Formula = "=MAXA(A1:A3)";
+
+        Assert.Equal(5d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTreatTextInRangeAsZeroAndNumericTextAsNumber()
+    {
+        sheet.Cells["A1"].Value = "abc"; // -> 0
+        sheet.Cells["A2"].Value = "15";  // -> 15
+        sheet.Cells["A3"].Value = 10;
+
+        sheet.Cells["B1"].Formula = "=MAXA(A1:A3)";
+
+        Assert.Equal(15d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCountDirectLogicalAndTextArguments()
+    {
+        sheet.Cells["A1"].Formula = "=MAXA(1=1, \"7\", 1=2)"; // TRUE, "7", FALSE -> 7
+        Assert.Equal(7d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroWhenNoValues()
+    {
+        sheet.Cells["A1"].Value = null; // empty
+        sheet.Cells["A2"].Value = "";  // empty string -> Empty
+
+        sheet.Cells["B1"].Formula = "=MAXA(A1:A2)";
+
+        Assert.Equal(0d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldPropagateErrors()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=A1/A2"; // #DIV/0!
+
+        sheet.Cells["B1"].Formula = "=MAXA(A1:A3)";
+
+        Assert.Equal(CellError.Div0, sheet.Cells["B1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/MaxFunctionTests.cs

@@ -0,0 +1,69 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MaxFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldReturnLargestValueFromNumbers()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 7;
+        sheet.Cells["A3"].Value = 9;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = 2;
+
+        sheet.Cells["B1"].Formula = "=MAX(A1:A5)";
+
+        Assert.Equal(27d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnLargestValueFromRangeAndLiteral()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 7;
+        sheet.Cells["A3"].Value = 9;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = 2;
+
+        sheet.Cells["B1"].Formula = "=MAX(A1:A5,30)";
+
+        Assert.Equal(30d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldIgnoreNonNumericInRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = null;
+
+        sheet.Cells["B1"].Formula = "=MAX(A1:A5)";
+
+        Assert.Equal(27d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTreatDirectLogicalAndNumericStringsAsNumbers()
+    {
+        sheet.Cells["A1"].Formula = "=MAX(\"15\", 5, 10)";
+        Assert.Equal(15d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroWhenNoNumbers()
+    {
+        sheet.Cells["A1"].Value = "a";
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=MAX(A1:A2)";
+
+        Assert.Equal(0d, sheet.Cells["A3"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/MidFunctionTests.cs

@@ -0,0 +1,51 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MidFunctionTests
+{
+    [Fact]
+    public void Mid_Start1_Take5()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Fluid Flow"; // length 10
+        sheet.Cells["B1"].Formula = "=MID(A2,1,5)";
+        Assert.Equal("Fluid", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Mid_Start7_Take20_Clamped()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Fluid Flow"; // length 10
+        sheet.Cells["B1"].Formula = "=MID(A2,7,20)";
+        Assert.Equal("Flow", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Mid_StartBeyondLength_ReturnsEmpty()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Fluid Flow"; // length 10
+        sheet.Cells["B1"].Formula = "=MID(A2,20,5)";
+        Assert.Equal(string.Empty, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Mid_StartLessThan1_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Fluid Flow";
+        sheet.Cells["B1"].Formula = "=MID(A2,0,5)";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+
+    [Fact]
+    public void Mid_NegativeNumChars_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Fluid Flow";
+        sheet.Cells["B1"].Formula = "=MID(A2,1,-1)";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/MinAllFunctionTests.cs

@@ -0,0 +1,64 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MinAllFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldEvaluateLogicalValuesInRange()
+    {
+        sheet.Cells["A1"].Value = true;   // 1
+        sheet.Cells["A2"].Value = false;  // 0
+        sheet.Cells["A3"].Value = 5;      // 5
+
+        sheet.Cells["B1"].Formula = "=MINA(A1:A3)";
+
+        Assert.Equal(0d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTreatTextInRangeAsZeroAndNumericTextAsNumber()
+    {
+        sheet.Cells["A1"].Value = "abc"; // -> 0
+        sheet.Cells["A2"].Value = "15";  // -> 15
+        sheet.Cells["A3"].Value = 10;
+
+        sheet.Cells["B1"].Formula = "=MINA(A1:A3)";
+
+        Assert.Equal(0d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCountDirectLogicalAndTextArguments()
+    {
+        sheet.Cells["A1"].Formula = "=MINA(1=1, \"7\", 1=2)"; // TRUE, "7", FALSE -> min is 0
+        Assert.Equal(0d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroWhenNoValues()
+    {
+        sheet.Cells["A1"].Value = null; // empty
+        sheet.Cells["A2"].Value = "";  // empty string -> Empty
+
+        sheet.Cells["B1"].Formula = "=MINA(A1:A2)";
+
+        Assert.Equal(0d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldPropagateErrors()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=A1/A2"; // #DIV/0!
+
+        sheet.Cells["B1"].Formula = "=MINA(A1:A3)";
+
+        Assert.Equal(CellError.Div0, sheet.Cells["B1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/MinFunctionTests.cs

@@ -0,0 +1,69 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MinFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldReturnSmallestValueFromNumbers()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 7;
+        sheet.Cells["A3"].Value = 9;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = 2;
+
+        sheet.Cells["B1"].Formula = "=MIN(A1:A5)";
+
+        Assert.Equal(2d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnSmallestValueFromRangeAndLiteral()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 7;
+        sheet.Cells["A3"].Value = 9;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = 2;
+
+        sheet.Cells["B1"].Formula = "=MIN(A1:A5,1)";
+
+        Assert.Equal(1d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldIgnoreNonNumericInRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = "text";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Value = 27;
+        sheet.Cells["A5"].Value = null;
+
+        sheet.Cells["B1"].Formula = "=MIN(A1:A5)";
+
+        Assert.Equal(10d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTreatDirectNumericStringsAsNumbers()
+    {
+        sheet.Cells["A1"].Formula = "=MIN(\"15\", 5, 10)";
+        Assert.Equal(5d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnZeroWhenNoNumbers()
+    {
+        sheet.Cells["A1"].Value = "a";
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=MIN(A1:A2)";
+
+        Assert.Equal(0d, sheet.Cells["A3"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/MinuteFunctionTests.cs

@@ -0,0 +1,34 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MinuteFunctionTests
+{
+    [Fact]
+    public void Minute_FromFraction_ReturnsMinutes()
+    {
+        var sheet = new Sheet(10, 10);
+        // 0.78125 = 18:45 -> minutes 45
+        sheet.Cells["A1"].Data = CellData.FromNumber(0.78125);
+        sheet.Cells["B1"].Formula = "=MINUTE(A1)";
+        Assert.Equal(45, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Minute_FromDateTimeValue_ReturnsMinute()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Data = CellData.FromDate(new System.DateTime(2011, 7, 18, 7, 45, 0));
+        sheet.Cells["B2"].Formula = "=MINUTE(A2)";
+        Assert.Equal(45, sheet.Cells["B2"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Minute_FromDateOnly_ReturnsZero()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Data = CellData.FromDate(new System.DateTime(2012, 4, 21));
+        sheet.Cells["B3"].Formula = "=MINUTE(A3)";
+        Assert.Equal(0, sheet.Cells["B3"].Data.GetValueOrDefault<double>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/MonthFunctionTests.cs

@@ -0,0 +1,31 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class MonthFunctionTests
+{
+    [Fact]
+    public void Month_FromDateSerial_ReturnsMonth()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=MONTH(VALUE(\"2011-04-15\"))";
+        Assert.Equal(4, sheet.Cells["A1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Month_FromDateValue_ReturnsMonth()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2011, 4, 15));
+        sheet.Cells["B1"].Formula = "=MONTH(A1)";
+        Assert.Equal(4, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Month_InvalidText_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=MONTH(\"abc\")";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/NotFunctionTests.cs

@@ -0,0 +1,200 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class NotFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithTrueValue()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+    
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithEmptyStringAsError()
+    {
+        sheet.Cells["A1"].Value = "";
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithFalseValue()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(true, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithNumericValue()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithZeroValue()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(true, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithStringValue()
+    {
+        sheet.Cells["A1"].Value = "test";
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithEmptyValue()
+    {
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(true, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithComparison()
+    {
+        sheet.Cells["A1"].Value = 50;
+        sheet.Cells["A2"].Formula = "=NOT(A1>100)";
+
+        Assert.Equal(true, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithTrueComparison()
+    {
+        sheet.Cells["A1"].Value = 150;
+        sheet.Cells["A2"].Formula = "=NOT(A1>100)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForNotFunctionWithNoArguments()
+    {
+        sheet.Cells["A1"].Formula = "=NOT()";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForNotFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=NOT(A1,A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithRangeExpression()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Formula = "=NOT(A1:A1)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithDecimalValue()
+    {
+        sheet.Cells["A1"].Value = 0.5m;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithNegativeValue()
+    {
+        sheet.Cells["A1"].Value = -5;
+        sheet.Cells["A2"].Formula = "=NOT(A1)";
+
+        Assert.Equal(false, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionInIfStatement()
+    {
+        sheet.Cells["A1"].Value = 50;
+        sheet.Cells["A2"].Formula = "=IF(NOT(A1>100),\"Valid\",\"Invalid\")";
+
+        Assert.Equal("Valid", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionInIfStatementWithFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 150;
+        sheet.Cells["A2"].Formula = "=IF(NOT(A1>100),\"Valid\",\"Invalid\")";
+
+        Assert.Equal("Invalid", sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithProvidedExample1()
+    {
+        sheet.Cells["A2"].Value = 50;
+        sheet.Cells["A3"].Formula = "=NOT(A2>100)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithProvidedExample2()
+    {
+        sheet.Cells["A2"].Value = 50;
+        sheet.Cells["A3"].Formula = "=IF(AND(NOT(A2>1),NOT(A2<100)),A2,\"The value is out of range\")";
+
+        Assert.Equal("The value is out of range", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithProvidedExample3()
+    {
+        sheet.Cells["A3"].Value = 100;
+        sheet.Cells["A4"].Formula = "=IF(OR(NOT(A3<0),NOT(A3>50)),A3,\"The value is out of range\")";
+
+        Assert.Equal(100d, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithNestedLogicalFunctions()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=NOT(AND(A1,A2))";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateNotFunctionWithOrFunction()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=NOT(OR(A1,A2))";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/NowFunctionTests.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class NowFunctionTests
+{
+    [Fact]
+    public void Now_ReturnsCurrentDateTime()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=NOW()";
+        var dt = sheet.Cells["A1"].Data.GetValueOrDefault<System.DateTime>();
+        Assert.Equal(System.DateTime.Today, dt.Date);
+    }
+
+    [Fact]
+    public void Now_MinusToday_IsFractionalDayBetween0And1()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=NOW()-TODAY()";
+        var serial = sheet.Cells["A1"].Data.GetValueOrDefault<double>();
+        Assert.True(serial >= 0 && serial < 1);
+    }
+
+    [Fact]
+    public void Now_PlusSevenDays_MinusToday_IsBetweenSevenAndEight()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=NOW()+7 - TODAY()";
+        var serial = sheet.Cells["A1"].Data.GetValueOrDefault<double>();
+        Assert.True(serial >= 7 && serial < 8);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/OrFunctionTests.cs

@@ -0,0 +1,216 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class OrFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithAllTrueValues()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithOneTrueValue()
+    {
+        sheet.Cells["A1"].Value = true;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithAllFalseValues()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(false, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithNumericValues()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=OR(A1>1,A2<100)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithZeroAsFalse()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Value = 1;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithBothZeroAsFalse()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Value = 0;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(false, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithNonZeroAsTrue()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithStringValues()
+    {
+        sheet.Cells["A1"].Value = "test";
+        sheet.Cells["A2"].Value = "hello";
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithStringValueAndNumber()
+    {
+        sheet.Cells["A1"].Value = "2";
+        sheet.Cells["A2"].Value = "hello";
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithEmptyStringAsFalse()
+    {
+        sheet.Cells["A2"].Value = "2";
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithBothEmptyStringsAsFalse()
+    {
+        sheet.Cells["A3"].Formula = "=OR(A1,A2)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Formula = "=OR(A1,A2,A3)";
+
+        Assert.Equal(true, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithAllFalseInMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = false;
+        sheet.Cells["A3"].Value = false;
+        sheet.Cells["A4"].Formula = "=OR(A1,A2,A3)";
+
+        Assert.Equal(false, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForEmptyOrFunction()
+    {
+        sheet.Cells["A1"].Formula = "=OR()";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithRangeExpression()
+    {
+        sheet.Cells["A1"].Value = false;
+        sheet.Cells["A2"].Value = true;
+        sheet.Cells["A3"].Formula = "=OR(A1:A2)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithMixedTypes()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Value = "";
+        sheet.Cells["A3"].Value = true;
+        sheet.Cells["A4"].Formula = "=OR(A1,A2,A3)";
+
+        Assert.Equal(true, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionInIfStatement()
+    {
+        sheet.Cells["A1"].Value = 5;
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Formula = "=IF(OR(A1>1,A2<100),A1,\"Out of range\")";
+
+        Assert.Equal(5d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionInIfStatementWithFalseCondition()
+    {
+        sheet.Cells["A1"].Value = 0;
+        sheet.Cells["A2"].Value = 150;
+        sheet.Cells["A3"].Formula = "=IF(OR(A1>1,A2<100),A1,\"Out of range\")";
+
+        Assert.Equal("Out of range", sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithProvidedExample1()
+    {
+        sheet.Cells["A2"].Value = 50;
+        sheet.Cells["A3"].Formula = "=OR(A2>1,A2<100)";
+
+        Assert.Equal(true, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithProvidedExample2()
+    {
+        sheet.Cells["A2"].Value = 5;
+        sheet.Cells["A3"].Value = 25;
+        sheet.Cells["A4"].Formula = "=IF(OR(A2>1,A2<100),A3,\"The value is out of range\")";
+
+        Assert.Equal(25d, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateOrFunctionWithProvidedExample3()
+    {
+        sheet.Cells["A2"].Value = 75;
+        sheet.Cells["A3"].Formula = "=IF(OR(A2<0,A2>50),A2,\"The value is out of range\")";
+
+        Assert.Equal(75d, sheet.Cells["A3"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/ProperFunctionTests.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ProperFunctionTests
+{
+    [Fact]
+    public void Proper_TitleCase_Simple()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "this is a TITLE";
+        sheet.Cells["B1"].Formula = "=PROPER(A2)";
+        Assert.Equal("This Is A Title", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Proper_KeepsHyphenation()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "2-way street";
+        sheet.Cells["B1"].Formula = "=PROPER(A3)";
+        Assert.Equal("2-Way Street", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Proper_AlnumBoundary()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A4"].Value = "76BudGet";
+        sheet.Cells["B1"].Formula = "=PROPER(A4)";
+        Assert.Equal("76Budget", sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RandBetweenFunctionTests.cs

@@ -0,0 +1,31 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RandBetweenFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Theory]
+    [InlineData(1, 100)]
+    [InlineData(-1, 1)]
+    [InlineData(0, 0)]
+    public void RandBetween_ShouldReturnInclusiveRange(int bottom, int top)
+    {
+        static string Tok(int n) => n < 0 ? $"0{n}" : n.ToString();
+        sheet.Cells["A1"].Formula = $"=RANDBETWEEN({Tok(bottom)},{Tok(top)})";
+        var v = sheet.Cells["A1"].Value;
+        Assert.IsType<double>(v); // numeric stored as double
+        var d = (double)v;
+        Assert.True(d >= bottom && d <= top);
+    }
+
+    [Fact]
+    public void RandBetween_ShouldReturnNumError_WhenBottomGreaterThanTop()
+    {
+        sheet.Cells["A1"].Formula = "=RANDBETWEEN(5,1)";
+        Assert.Equal(CellError.Num, sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/RandFunctionTests.cs

@@ -0,0 +1,36 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RandFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void Rand_ShouldReturnInRangeZeroToOneExclusiveOfOne()
+    {
+        sheet.Cells["A1"].Formula = "=RAND()";
+        var v = sheet.Cells["A1"].Value;
+        Assert.IsType<double>(v);
+        var d = (double)v;
+        Assert.True(d >= 0d && d < 1d);
+    }
+
+    [Fact]
+    public void Rand_RecalculatesOnFormulaReassignment()
+    {
+        sheet.Cells["A1"].Formula = "=RAND()";
+        var d1 = (double)sheet.Cells["A1"].Value;
+        sheet.Cells["A1"].Formula = "=RAND()"; // force recalc
+        var d2 = (double)sheet.Cells["A1"].Value;
+        // It's possible (though unlikely) to be equal; allow a retry window
+        if (d1 == d2)
+        {
+            sheet.Cells["A1"].Formula = "=RAND()";
+            d2 = (double)sheet.Cells["A1"].Value;
+        }
+        Assert.True(d1 != d2 || (d1 >= 0d && d1 < 1d));
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/RangeSelectionItemTests.cs

@@ -0,0 +1,132 @@
+using Bunit;
+using System.Linq;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RangeSelectionItemTests : TestContext
+{
+    private readonly Sheet sheet = new(4, 4);
+
+    [Fact]
+    public void RangeSelectionItem_ShouldCalculateCorrectMaskForMergedCells()
+    {
+        // Arrange
+        var mergedRange = RangeRef.Parse("B1:C1");
+        sheet.MergedCells.Add(mergedRange);
+        
+        // Select a range that overlaps with the merged cell
+        var selectionRange = RangeRef.Parse("A1:D1");
+        sheet.Selection.Select(selectionRange);
+        
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<RangeSelectionItem>(parameters => parameters
+            .Add(p => p.Range, selectionRange)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Cell, sheet.Selection.Cell) // This should be A1 (the active cell)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-range");
+        Assert.NotNull(element);
+        
+        // The style should include mask properties that account for the merged cell
+        var style = element.GetAttribute("style");
+        Assert.NotNull(style);
+        Assert.Contains("mask-size", style);
+        Assert.Contains("mask-position", style);
+    }
+
+    [Fact]
+    public void RangeSelectionItem_ShouldHandleNonMergedCellsCorrectly()
+    {
+        // Arrange
+        var selectionRange = RangeRef.Parse("A1:B2");
+        sheet.Selection.Select(selectionRange);
+        
+        var context = new MockVirtualGridContext();
+
+        // Act
+        var cut = RenderComponent<RangeSelectionItem>(parameters => parameters
+            .Add(p => p.Range, selectionRange)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Cell, sheet.Selection.Cell)
+            .Add(p => p.Context, context));
+
+        // Assert
+        var element = cut.Find(".rz-spreadsheet-selection-range");
+        Assert.NotNull(element);
+        
+        var style = element.GetAttribute("style");
+        Assert.NotNull(style);
+        Assert.Contains("transform", style);
+        Assert.Contains("width", style);
+        Assert.Contains("height", style);
+    }
+
+    [Fact]
+    public void RangeSelectionItem_ShouldHandleMergedCellsAcrossFrozenColumnBoundary()
+    {
+        // Arrange
+        sheet.Columns.Frozen = 1; // Freeze first column
+        var mergedRange = RangeRef.Parse("A1:B1"); // Merged cell spans across frozen boundary
+        sheet.MergedCells.Add(mergedRange);
+        
+        // Select the merged cell
+        sheet.Selection.Select(mergedRange);
+        
+        var context = new MockVirtualGridContext();
+
+        // Get the split ranges (frozen and non-frozen parts)
+        var ranges = sheet.GetRanges(mergedRange).ToList();
+        Assert.Equal(2, ranges.Count); // Should be split into 2 parts
+
+        // Test the frozen part (A1:A1)
+        var frozenRange = ranges.First(r => r.FrozenColumn);
+        var frozenCut = RenderComponent<RangeSelectionItem>(parameters => parameters
+            .Add(p => p.Range, frozenRange.Range)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Cell, sheet.Selection.Cell)
+            .Add(p => p.Context, context)
+            .Add(p => p.FrozenColumn, frozenRange.FrozenColumn)
+            .Add(p => p.FrozenRow, frozenRange.FrozenRow)
+            .Add(p => p.Top, frozenRange.Top)
+            .Add(p => p.Left, frozenRange.Left)
+            .Add(p => p.Bottom, frozenRange.Bottom)
+            .Add(p => p.Right, frozenRange.Right));
+
+        var frozenElement = frozenCut.Find(".rz-spreadsheet-selection-range");
+        Assert.NotNull(frozenElement);
+        Assert.Contains("rz-spreadsheet-frozen-column", frozenElement.ClassName);
+        
+        var frozenStyle = frozenElement.GetAttribute("style");
+        Assert.NotNull(frozenStyle);
+        Assert.Contains("mask-size", frozenStyle);
+        Assert.Contains("mask-position", frozenStyle);
+
+        // Test the non-frozen part (B1:B1)
+        var nonFrozenRange = ranges.First(r => !r.FrozenColumn);
+        var nonFrozenCut = RenderComponent<RangeSelectionItem>(parameters => parameters
+            .Add(p => p.Range, nonFrozenRange.Range)
+            .Add(p => p.Sheet, sheet)
+            .Add(p => p.Cell, sheet.Selection.Cell)
+            .Add(p => p.Context, context)
+            .Add(p => p.FrozenColumn, nonFrozenRange.FrozenColumn)
+            .Add(p => p.FrozenRow, nonFrozenRange.FrozenRow)
+            .Add(p => p.Top, nonFrozenRange.Top)
+            .Add(p => p.Left, nonFrozenRange.Left)
+            .Add(p => p.Bottom, nonFrozenRange.Bottom)
+            .Add(p => p.Right, nonFrozenRange.Right));
+
+        var nonFrozenElement = nonFrozenCut.Find(".rz-spreadsheet-selection-range");
+        Assert.NotNull(nonFrozenElement);
+        Assert.DoesNotContain("rz-spreadsheet-frozen-column", nonFrozenElement.ClassName);
+        
+        var nonFrozenStyle = nonFrozenElement.GetAttribute("style");
+        Assert.NotNull(nonFrozenStyle);
+        Assert.Contains("mask-size", nonFrozenStyle);
+        Assert.Contains("mask-position", nonFrozenStyle);
+    }
+} 

Radzen.Blazor.Tests/Spreadsheet/ReplaceFunctionTests.cs

@@ -0,0 +1,51 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ReplaceFunctionTests
+{
+    [Fact]
+    public void Replace_Middle_WithAsterisk()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "abcdefghijk";
+        sheet.Cells["B1"].Formula = "=REPLACE(A2,6,5,\"*\")";
+        Assert.Equal("abcde*k", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Replace_LastTwoDigits_With10()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "2009";
+        sheet.Cells["B1"].Formula = "=REPLACE(A3,3,2,\"10\")";
+        Assert.Equal("2010", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Replace_FirstThree_WithAt()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A4"].Value = "123456";
+        sheet.Cells["B1"].Formula = "=REPLACE(A4,1,3,\"@\")";
+        Assert.Equal("@456", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Replace_StartBeyond_Appends()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "abc";
+        sheet.Cells["B1"].Formula = "=REPLACE(A1,10,2,\"XYZ\")";
+        Assert.Equal("abcXYZ", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Replace_InvalidStartNum_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "abc";
+        sheet.Cells["B1"].Formula = "=REPLACE(A1,0,2,\"X\")";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/ReptFunctionTests.cs

@@ -0,0 +1,47 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ReptFunctionTests
+{
+    [Fact]
+    public void Rept_Basic()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=REPT(\"*-\",3)";
+        Assert.Equal("*-*-*-", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Rept_DashesTen()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=REPT(\"-\",10)";
+        Assert.Equal("----------", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Rept_ZeroTimes_ReturnsEmpty()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=REPT(\"x\",0)";
+        Assert.Equal(string.Empty, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Rept_Negative_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=REPT(\"x\",-1)";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+
+    [Fact]
+    public void Rept_Overflow_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        // text length 2 * 20000 = 40000 > 32767
+        sheet.Cells["A1"].Formula = "=REPT(\"ab\",20000)";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RightFunctionTests.cs

@@ -0,0 +1,42 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RightFunctionTests
+{
+    [Fact]
+    public void Right_WithCount_ReturnsSuffix()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Sale Price";
+        sheet.Cells["B1"].Formula = "=RIGHT(A2,5)";
+        Assert.Equal("Price", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Right_OmittedCount_DefaultsToOne()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "Stock Number";
+        sheet.Cells["B1"].Formula = "=RIGHT(A3)";
+        Assert.Equal("r", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Right_CountExceedsLength_ReturnsWhole()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Hi";
+        sheet.Cells["B1"].Formula = "=RIGHT(A1,5)";
+        Assert.Equal("Hi", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Right_NegativeCount_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Test";
+        sheet.Cells["B1"].Formula = "=RIGHT(A1,-1)";
+        Assert.Equal(CellError.Value, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RoundDownFunctionTests.cs

@@ -0,0 +1,39 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RoundDownFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldRoundDownToZeroDecimalPlaces()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDDOWN(3.2,0)";
+        Assert.Equal(3d, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Formula = "=ROUNDDOWN(76.9,0)";
+        Assert.Equal(76d, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundDownToSpecifiedDecimalPlaces()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDDOWN(3.14159,3)";
+        Assert.Equal(3.141, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundDownNegativeNumbersTowardZero()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDDOWN(0-3.14159,1)";
+        Assert.Equal(-3.1, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundDownToLeftOfDecimalWhenNegativeDigits()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDDOWN(31415.92654,0-2)";
+        Assert.Equal(31400d, sheet.Cells["A1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RoundFunctionTests.cs

@@ -0,0 +1,41 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RoundFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldRoundToOneDecimalPlace()
+    {
+        sheet.Cells["A1"].Formula = "=ROUND(2.15,1)";
+        Assert.Equal(2.2, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Formula = "=ROUND(2.149,1)";
+        Assert.Equal(2.1, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundNegativeWithAwayFromZeroMidpoint()
+    {
+        sheet.Cells["A1"].Formula = "=ROUND(0-1.475,2)";
+        Assert.Equal(-1.48, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundWithNegativeDigits()
+    {
+        sheet.Cells["A1"].Formula = "=ROUND(21.5,0-1)";
+        Assert.Equal(20d, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Formula = "=ROUND(626.3,0-3)";
+        Assert.Equal(1000d, sheet.Cells["A2"].Value);
+
+        sheet.Cells["A3"].Formula = "=ROUND(1.98,0-1)";
+        Assert.Equal(0d, sheet.Cells["A3"].Value);
+
+        sheet.Cells["A4"].Formula = "=ROUND(0-50.55,0-2)";
+        Assert.Equal(-100d, sheet.Cells["A4"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RoundUpFunctionTests.cs

@@ -0,0 +1,39 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RoundUpFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldRoundUpToZeroDecimalPlaces()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDUP(3.2,0)";
+        Assert.Equal(4d, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Formula = "=ROUNDUP(76.9,0)";
+        Assert.Equal(77d, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundUpToSpecifiedDecimalPlaces()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDUP(3.14159,3)";
+        Assert.Equal(3.142, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundUpNegativeNumbersAwayFromZero()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDUP(0-3.14159,1)";
+        Assert.Equal(-3.2, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRoundUpToLeftOfDecimalWhenNegativeDigits()
+    {
+        sheet.Cells["A1"].Formula = "=ROUNDUP(31415.92654,0-2)";
+        Assert.Equal(31500d, sheet.Cells["A1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RowColumnCommandTests.cs

@@ -0,0 +1,301 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RowColumnCommandTests
+{
+    [Fact]
+    public void DeleteRowsCommand_SingleRow_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(4, 3);
+        sheet.Cells[0, 0].Value = "R1";
+        sheet.Cells[1, 0].Value = "R2";
+        sheet.Cells[2, 0].Value = "R3";
+        sheet.Cells[3, 0].Value = "R4";
+
+        var cmd = new DeleteRowsCommand(sheet, 1, 1);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(3, sheet.RowCount);
+        Assert.Equal("R1", sheet.Cells[0, 0].Value);
+        Assert.Equal("R3", sheet.Cells[1, 0].Value);
+        Assert.Equal("R4", sheet.Cells[2, 0].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(4, sheet.RowCount);
+        Assert.Equal("R1", sheet.Cells[0, 0].Value);
+        Assert.Equal("R2", sheet.Cells[1, 0].Value);
+        Assert.Equal("R3", sheet.Cells[2, 0].Value);
+        Assert.Equal("R4", sheet.Cells[3, 0].Value);
+    }
+
+    [Fact]
+    public void DeleteRowsCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(6, 3);
+        sheet.Cells[0, 0].Value = "R1";
+        sheet.Cells[1, 0].Value = "R2";
+        sheet.Cells[2, 0].Value = "R3";
+        sheet.Cells[3, 0].Value = "R4";
+        sheet.Cells[4, 0].Value = "R5";
+        sheet.Cells[5, 0].Value = "R6";
+
+        var cmd = new DeleteRowsCommand(sheet, 1, 3); // delete rows 2..4
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(3, sheet.RowCount);
+        Assert.Equal("R1", sheet.Cells[0, 0].Value);
+        Assert.Equal("R5", sheet.Cells[1, 0].Value);
+        Assert.Equal("R6", sheet.Cells[2, 0].Value);
+
+        sheet.Commands.Undo();
+        Assert.Equal(6, sheet.RowCount);
+        Assert.Equal("R1", sheet.Cells[0, 0].Value);
+        Assert.Equal("R2", sheet.Cells[1, 0].Value);
+        Assert.Equal("R3", sheet.Cells[2, 0].Value);
+        Assert.Equal("R4", sheet.Cells[3, 0].Value);
+        Assert.Equal("R5", sheet.Cells[4, 0].Value);
+        Assert.Equal("R6", sheet.Cells[5, 0].Value);
+    }
+
+    [Fact]
+    public void InsertRowAfterCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(4, 3);
+        sheet.Cells[1, 0].Value = 1d; // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+
+        var cmd = new InsertRowAfterCommand(sheet, 1); // after row 1 -> insert at index 2
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(5, sheet.RowCount);
+        // Inserting after row 1 does not move row 1; A2 and B2 stay
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 1].Formula);
+        Assert.Equal(11d, sheet.Cells[1, 1].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(4, sheet.RowCount);
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 1].Formula);
+    }
+
+    [Fact]
+    public void InsertRowBeforeCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(4, 3);
+        sheet.Cells[1, 0].Value = 1d; // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+
+        var cmd = new InsertRowBeforeCommand(sheet, 1); // before row 1 -> insert at index 1
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(5, sheet.RowCount);
+        // value shifted down (A2 becomes A3)
+        Assert.Equal(1d, sheet.Cells[2, 0].Value);
+        // formula shifted down and reference updated A2->A3
+        Assert.Equal("=A3+10", sheet.Cells[2, 1].Formula);
+        Assert.Equal(11d, sheet.Cells[2, 1].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(4, sheet.RowCount);
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 1].Formula);
+    }
+
+    [Fact]
+    public void InsertRowAfterCommand_WithMultiSelection_InsertsAfterLastRow()
+    {
+        var sheet = new Sheet(6, 2);
+        // Mark rows with their index in A
+        for (int r = 0; r < 6; r++) sheet.Cells[r, 0].Value = r + 1;
+
+        // Simulate selection rows 1..3 (0-based), last is 3 -> insert after 3 at index 4
+        var cmd = new InsertRowAfterCommand(sheet, 3);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(7, sheet.RowCount);
+        // Inserted row at index 4 is empty
+        Assert.Null(sheet.Cells[4, 0].Value);
+        // Row 5 takes previous row 4 value (5)
+        Assert.Equal(5d, sheet.Cells[5, 0].Value);
+        // Row 6 takes previous row 5 value (6)
+        Assert.Equal(6d, sheet.Cells[6, 0].Value);
+
+        sheet.Commands.Undo();
+        Assert.Equal(6, sheet.RowCount);
+        Assert.Equal(5d, sheet.Cells[4, 0].Value);
+    }
+
+    [Fact]
+    public void InsertRowBeforeCommand_WithMultiSelection_InsertsBeforeFirstRow()
+    {
+        var sheet = new Sheet(6, 2);
+        for (int r = 0; r < 6; r++) sheet.Cells[r, 0].Value = r + 1;
+
+        // Simulate selection rows 2..4 (first is 2) -> insert at index 2
+        var cmd = new InsertRowBeforeCommand(sheet, 2);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(7, sheet.RowCount);
+        // Inserted row at index 2 is empty
+        Assert.Null(sheet.Cells[2, 0].Value);
+        // Row 3 takes previous row 2 value (3)
+        Assert.Equal(3d, sheet.Cells[3, 0].Value);
+
+        sheet.Commands.Undo();
+        Assert.Equal(6, sheet.RowCount);
+        Assert.Equal(3d, sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void InsertColumnAfterCommand_WithMultiSelection_InsertsAfterLastColumn()
+    {
+        var sheet = new Sheet(2, 6);
+        // Mark columns with their index in row 1
+        for (int c = 0; c < 6; c++) sheet.Cells[0, c].Value = c + 1;
+
+        // Simulate selection columns 1..3 (last is 3) -> insert after col 3 at index 4
+        var cmd = new InsertColumnAfterCommand(sheet, 3);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(7, sheet.ColumnCount);
+        // Inserted column at index 4 is empty
+        Assert.Null(sheet.Cells[0, 4].Value);
+        // Column 5 takes previous column 4 value (5)
+        Assert.Equal(5d, sheet.Cells[0, 5].Value);
+
+        sheet.Commands.Undo();
+        Assert.Equal(6, sheet.ColumnCount);
+        Assert.Equal(5d, sheet.Cells[0, 4].Value);
+    }
+
+    [Fact]
+    public void InsertColumnBeforeCommand_WithMultiSelection_InsertsBeforeFirstColumn()
+    {
+        var sheet = new Sheet(2, 6);
+        for (int c = 0; c < 6; c++) sheet.Cells[0, c].Value = c + 1;
+
+        // Simulate selection columns 2..4 (first is 2) -> insert at index 2
+        var cmd = new InsertColumnBeforeCommand(sheet, 2);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(7, sheet.ColumnCount);
+        // Inserted column at index 2 is empty
+        Assert.Null(sheet.Cells[0, 2].Value);
+        // Column 3 takes previous column 2 value (3)
+        Assert.Equal(3d, sheet.Cells[0, 3].Value);
+
+        sheet.Commands.Undo();
+        Assert.Equal(6, sheet.ColumnCount);
+        Assert.Equal(3d, sheet.Cells[0, 2].Value);
+    }
+
+    [Fact]
+    public void DeleteColumnsCommand_SingleColumn_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(3, 4);
+        sheet.Cells[0, 0].Value = "A";
+        sheet.Cells[0, 1].Value = "B";
+        sheet.Cells[0, 2].Value = "C";
+        sheet.Cells[0, 3].Value = "D";
+
+        var cmd = new DeleteColumnsCommand(sheet, 1, 1);
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(3, sheet.ColumnCount);
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("C", sheet.Cells[0, 1].Value);
+        Assert.Equal("D", sheet.Cells[0, 2].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(4, sheet.ColumnCount);
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("B", sheet.Cells[0, 1].Value);
+        Assert.Equal("C", sheet.Cells[0, 2].Value);
+        Assert.Equal("D", sheet.Cells[0, 3].Value);
+    }
+
+    [Fact]
+    public void DeleteColumnsCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(3, 6);
+        sheet.Cells[0, 0].Value = "A";
+        sheet.Cells[0, 1].Value = "B";
+        sheet.Cells[0, 2].Value = "C";
+        sheet.Cells[0, 3].Value = "D";
+        sheet.Cells[0, 4].Value = "E";
+        sheet.Cells[0, 5].Value = "F";
+
+        var cmd = new DeleteColumnsCommand(sheet, 1, 3); // delete B..D
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(3, sheet.ColumnCount);
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("E", sheet.Cells[0, 1].Value);
+        Assert.Equal("F", sheet.Cells[0, 2].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(6, sheet.ColumnCount);
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("B", sheet.Cells[0, 1].Value);
+        Assert.Equal("C", sheet.Cells[0, 2].Value);
+        Assert.Equal("D", sheet.Cells[0, 3].Value);
+        Assert.Equal("E", sheet.Cells[0, 4].Value);
+        Assert.Equal("F", sheet.Cells[0, 5].Value);
+    }
+
+    [Fact]
+    public void InsertColumnBeforeCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.Cells[1, 0].Value = 1d;   // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+
+        var cmd = new InsertColumnBeforeCommand(sheet, 0); // before A
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(6, sheet.ColumnCount);
+        // value shifted right (A2 becomes B2)
+        Assert.Equal(1d, sheet.Cells[1, 1].Value);
+        // formula shifted right and reference updated A2->B2
+        Assert.Equal("=B2+10", sheet.Cells[1, 2].Formula); // original B2 moved to C2
+        Assert.Equal(11d, sheet.Cells[1, 2].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(5, sheet.ColumnCount);
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 1].Formula);
+    }
+
+    [Fact]
+    public void InsertColumnAfterCommand_ExecuteAndUndo_RestoresState()
+    {
+        var sheet = new Sheet(5, 5);
+        sheet.Cells[1, 0].Value = 1d;   // A2
+        sheet.Cells[1, 1].Formula = "=A2+10"; // B2
+
+        var cmd = new InsertColumnAfterCommand(sheet, 0); // after A
+        Assert.True(sheet.Commands.Execute(cmd));
+
+        Assert.Equal(6, sheet.ColumnCount);
+        // value at A2 stays, but B2 moves to C2; formula references should update if referencing >= inserted column
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 2].Formula); // original B2 moved to C2
+        Assert.Equal(11d, sheet.Cells[1, 2].Value);
+
+        sheet.Commands.Undo();
+
+        Assert.Equal(5, sheet.ColumnCount);
+        Assert.Equal(1d, sheet.Cells[1, 0].Value);
+        Assert.Equal("=A2+10", sheet.Cells[1, 1].Formula);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/RowFunctionTests.cs

@@ -0,0 +1,38 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RowFunctionTests
+{
+    [Fact]
+    public void Row_OmittedReference_ReturnsCurrentRow()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["C10"].Formula = "=ROW()";
+        Assert.Equal(10d, sheet.Cells["C10"].Data.Value);
+    }
+
+    [Fact]
+    public void Row_SingleCellReference_ReturnsThatRow()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["A1"].Formula = "=ROW(C10)";
+        Assert.Equal(10d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Row_RangeReference_ReturnsTopLeftRow()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["B2"].Formula = "=ROW(C10:E10)";
+        Assert.Equal(10d, sheet.Cells["B2"].Data.Value);
+    }
+
+    [Fact]
+    public void Row_RangeReference_MultiRowAndColumn_IsError()
+    {
+        var sheet = new Sheet(20, 10);
+        sheet.Cells["B2"].Formula = "=ROW(C10:D20)";
+        Assert.Equal(CellError.Value, sheet.Cells["B2"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/RowsFunctionTests.cs

@@ -0,0 +1,30 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class RowsFunctionTests
+{
+    [Fact]
+    public void Rows_Range_ReturnsRowCount()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=ROWS(C1:E4)";
+        Assert.Equal(4d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Rows_SingleCell_ReturnsOne()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=ROWS(C10)";
+        Assert.Equal(1d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Rows_SingleRowRange_ReturnsOne()
+    {
+        var sheet = new Sheet(50, 20);
+        sheet.Cells["A1"].Formula = "=ROWS(C10:E10)";
+        Assert.Equal(1d, sheet.Cells["A1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SearchFunctionTests.cs

@@ -0,0 +1,65 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SearchFunctionTests
+{
+    [Fact]
+    public void Search_SimpleCharacter()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=SEARCH(\"n\",\"printer\")";
+        Assert.Equal(4d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Search_Substring()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=SEARCH(\"base\",\"database\")";
+        Assert.Equal(5d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Search_StartNum_SkipsPrefix()
+    {
+        var sheet = new Sheet(10, 30);
+        sheet.Cells["A1"].Value = "AYF0093.YoungMensApparel";
+        sheet.Cells["B1"].Formula = "=SEARCH(\"Y\",A1,8)";
+        Assert.Equal(9d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Search_Wildcards_QuestionAndAsterisk()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "Profit Margin";
+        sheet.Cells["B1"].Formula = "=SEARCH(\"M*r?in\",A1)";
+        Assert.Equal(8d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Search_TildeEscapesWildcards()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = "a*?c";
+        sheet.Cells["B1"].Formula = "=SEARCH(\"~*~?\",A1)";
+        Assert.Equal(2d, sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Search_StartNumInvalid_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=SEARCH(\"e\",\"printer\",0)";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+
+    [Fact]
+    public void Search_NotFound_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=SEARCH(\"zzz\",\"printer\")";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SecondFunctionTests.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SecondFunctionTests
+{
+    [Fact]
+    public void Second_FromDateTimeWithSeconds_ReturnsSeconds()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2020, 1, 1, 16, 48, 18));
+        sheet.Cells["B1"].Formula = "=SECOND(A1)";
+        Assert.Equal(18, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Second_FromDateTimeWithoutSeconds_ReturnsZero()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Data = CellData.FromDate(new System.DateTime(2020, 1, 1, 16, 48, 0));
+        sheet.Cells["B2"].Formula = "=SECOND(A2)";
+        Assert.Equal(0, sheet.Cells["B2"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Second_FromDateOnly_ReturnsZero()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Data = CellData.FromDate(new System.DateTime(2020, 1, 1));
+        sheet.Cells["B3"].Formula = "=SECOND(A3)";
+        Assert.Equal(0, sheet.Cells["B3"].Data.GetValueOrDefault<double>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SelectionCycleTests.cs

@@ -0,0 +1,287 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SelectionCycleTests
+{
+    readonly Sheet sheet = new(4, 4);
+
+    [Fact]
+    public void Should_MoveToTheNextHorizontalCell()
+    {
+        sheet.Selection.Select(RangeRef.Parse("A1:C1"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:C1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheFirstCellInTheRangeWhenAtLastColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("C1"), RangeRef.Parse("A1:C1"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:C1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextRowInTheRangeWhenAtLastColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("C1"), RangeRef.Parse("A1:C2"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:C2"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextVerticalCell()
+    {
+        sheet.Selection.Select(RangeRef.Parse("A1:A3"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A3"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheFirstCellInTheRangeWhenAtLastRow()
+    {
+        sheet.Selection.Select(CellRef.Parse("A3"), RangeRef.Parse("A1:A3"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A3"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextColumnInTheRangeWhenAtLastRow()
+    {
+        sheet.Selection.Select(CellRef.Parse("A3"), RangeRef.Parse("A1:B3"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:B3"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToThePreviousHorizontalCell()
+    {
+        sheet.Selection.Select(RangeRef.Parse("B1:C1"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("C1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("B1:C1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheLastCellInTheRangeWhenAtFirstColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"), RangeRef.Parse("A1:C1"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("C1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:C1"), sheet.Selection.Range);
+    }
+
+
+    [Fact]
+    public void Should_MoveToThePreviousRowInTheRangeWhenAtFirstColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"), RangeRef.Parse("A1:C2"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("C2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:C2"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextHorizontalCellIfOnlyOneCellIsSelected()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("B1:B1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextVerticalCellIfOnlyOneCellIsSelected()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A2:A2"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToThePreviousHorizontalCellIfOnlyOneCellIsSelected()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveToThePreviousVerticalCellIfOnlyOneCellIsSelected()
+    {
+        sheet.Selection.Select(CellRef.Parse("A2"));
+
+        var cell = sheet.Selection.Cycle(-1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_StayInTheSameCellIfTheFirstCellIsSelectedAndMovingBackwards()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_StayInTheSameCellIfTheLastCellIsSelectedAndMovingForwards()
+    {
+        sheet.Selection.Select(CellRef.Parse("D4"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("D4", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("D4:D4"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_StayInTheSameCellIfOnlyOneCellIsSelectedAndMovingUp()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Cycle(-1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_StayInTheSameCellIfOnlyOneCellIsSelectedAndMovingDown()
+    {
+        sheet.Selection.Select(CellRef.Parse("D4"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("D4", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("D4:D4"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_GoToTheFirstCellInTheSheetIfNoSelection()
+    {
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_GoToTheNextHorizontalMergedCellInTheSelectedRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(RangeRef.Parse("A1:D1"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:D1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_GoToThePreviousHorizontalMergedCellInTheSelectedRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("D1"), RangeRef.Parse("A1:D1"));
+
+        var cell = sheet.Selection.Cycle(0, -1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:D1"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_GoToTheNextVerticalMergedCellInTheSelectedRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A2:A3"));
+        sheet.Selection.Select(RangeRef.Parse("A1:A4"));
+
+        var cell = sheet.Selection.Cycle(1, 0);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A4"), sheet.Selection.Range);
+    }
+    [Fact]
+    public void Should_GoToThePreviousVerticalMergedCellInTheSelectedRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A2:A3"));
+        sheet.Selection.Select(CellRef.Parse("A4"), RangeRef.Parse("A1:A4"));
+
+        var cell = sheet.Selection.Cycle(-1, 0);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("A1:A4"), sheet.Selection.Range);
+    }
+
+    [Fact]
+    public void Should_MoveFromMergedCellToNextCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        var cell = sheet.Selection.Cycle(0, 1);
+
+        Assert.Equal("D1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+        Assert.Equal(RangeRef.Parse("D1:D1"), sheet.Selection.Range);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SelectionExtensionTests.cs

@@ -0,0 +1,445 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SelectionExtensionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void Extend_ShouldAppendNextHorizontalCellToRange()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAppendNextVerticalCellToRange()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B1:B2", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousHorizontalCellFromRange()
+    {
+        sheet.Selection.Select(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousVerticalCellFromRange()
+    {
+        sheet.Selection.Select(RangeRef.Parse("B1:B2"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAppendPreviousHorizontalCellToRange()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldFirstAppendAndThenSubtractNextHorizontalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldFirstAppendAndThenSubtractPreviousHorizontalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, -1);
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAppendPreviousVerticalCellToRange()
+    {
+        sheet.Selection.Select(CellRef.Parse("B2"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B2", sheet.Selection.Range.ToString());
+        Assert.Equal("B2", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldFirstAppendAndThenSubtractVerticalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("B2"));
+        sheet.Selection.Extend(1, 0);
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B2:B2", sheet.Selection.Range.ToString());
+        Assert.Equal("B2", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Select_ShouldSelectTheWholeMergedCellRangeWhenTheStartIsUsed()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Select_ShouldSelectTheWholeMergedCellRangeWhenTheEndIsUsed()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("C1"));
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextHorizontalCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:D1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextHorizontalCellAfterMergedCellAgain()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:E1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextMergedCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.MergedCells.Add(RangeRef.Parse("D1:E1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:E1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractNextMergedCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.MergedCells.Add(RangeRef.Parse("D1:E1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, 1);
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousMergedCellBeforeMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.MergedCells.Add(RangeRef.Parse("D1:E1"));
+        sheet.Selection.Select(CellRef.Parse("D1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:E1", sheet.Selection.Range.ToString());
+        Assert.Equal("D1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousMergedCellBeforeMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.MergedCells.Add(RangeRef.Parse("D1:E1"));
+        sheet.Selection.Select(CellRef.Parse("D1"));
+        sheet.Selection.Extend(0, -1);
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("D1:E1", sheet.Selection.Range.ToString());
+        Assert.Equal("D1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousHorizontalMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("D1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:D1", sheet.Selection.Range.ToString());
+        Assert.Equal("D1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubractPreviousMergedCellFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("D1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:D1", sheet.Selection.Range.ToString());
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("D1:D1", sheet.Selection.Range.ToString());
+        Assert.Equal("D1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractNextMergedCellFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(RangeRef.Parse("A1:C1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:A1", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousHorizontalCellBeforeMergedCellFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Extend(0, -1);
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousHorizontalCellBeforeMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextVerticalCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B1:B3", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextVerticalCellAfterMergedCellAgain()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(1, 0);
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B1:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldHandleMergedCellsConsistentlyWhenMovingLeft()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("D1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("B1:D1", sheet.Selection.Range.ToString());
+        Assert.Equal("D1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldHandleMergedCellsConsistentlyWhenMovingLeftFromStart()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldHandleMergedCellsConsistentlyWhenMovingLeftFromEnd()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("C1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldHandleMergedCellsConsistentlyWhenMovingLeftFromEndAndThenRight()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("C1"));
+        sheet.Selection.Extend(0, -1);
+        sheet.Selection.Extend(0, 1);
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldHandleMergedCellsConsistentlyWhenMovingLeftFromMiddle()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:D1"));
+        sheet.Selection.Select(CellRef.Parse("C1"));
+        sheet.Selection.Extend(0, -1);
+
+        Assert.Equal("A1:D1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddNextMergedRowAfterMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.MergedCells.Add(RangeRef.Parse("B3:B4"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B1:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractNextMergedRowAfterMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.MergedCells.Add(RangeRef.Parse("B3:B4"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Extend(1, 0);
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B2", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousMergedRowBeforeMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.MergedCells.Add(RangeRef.Parse("B3:B4"));
+        sheet.Selection.Select(CellRef.Parse("B3"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B3", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousMergedRowBeforeMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:B2"));
+        sheet.MergedCells.Add(RangeRef.Parse("B3:B4"));
+        sheet.Selection.Select(CellRef.Parse("B3"));
+        sheet.Selection.Extend(-1, 0);
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B3:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B3", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousVerticalMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Select(CellRef.Parse("B4"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B2:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B4", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousMergedRowFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Select(CellRef.Parse("B4"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B2:B4", sheet.Selection.Range.ToString());
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B4:B4", sheet.Selection.Range.ToString());
+        Assert.Equal("B4", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractNextMergedRowFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Select(RangeRef.Parse("B1:B3"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldSubtractPreviousVerticalCellBeforeMergedRowFromSelection()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Select(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Extend(-1, 0);
+        Assert.Equal("B1:B3", sheet.Selection.Range.ToString());
+
+        sheet.Selection.Extend(1, 0);
+
+        Assert.Equal("B2:B3", sheet.Selection.Range.ToString());
+        Assert.Equal("B2", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Extend_ShouldAddPreviousVerticalCellBeforeMergedRow()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B2:B3"));
+        sheet.Selection.Select(CellRef.Parse("B2"));
+        sheet.Selection.Extend(-1, 0);
+
+        Assert.Equal("B1:B3", sheet.Selection.Range.ToString());
+        Assert.Equal("B2", sheet.Selection.Cell.ToString());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SelectionMergingTests.cs

@@ -0,0 +1,71 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SelectionMergingTests
+{
+    readonly Sheet sheet = new(4, 4);
+
+    [Fact]
+    public void Should_SelectAllHorizontalCellsBetweenTheCurrentCellAndTheNewOne()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+        sheet.Selection.Merge(CellRef.Parse("C1"));
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectAllVerticalCellsBetweenTheCurrentCellAndTheNewOne()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+        sheet.Selection.Merge(CellRef.Parse("A3"));
+
+        Assert.Equal("A1:A3", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_ExtendRangeWhenMerginANewCell()
+    {
+        sheet.Selection.Select(RangeRef.Parse("A1:C1"));
+        sheet.Selection.Merge(CellRef.Parse("D2"));
+
+        Assert.Equal("A1:D2", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_AddTheEntireMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("A1"));
+        sheet.Selection.Merge(CellRef.Parse("B1"));
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_AddTheEntireMergedCellWhenSelectingIt()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+        sheet.Selection.Merge(CellRef.Parse("A1"));
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_ExpandSelectionToIncludeMergedCells()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("A1"));
+        sheet.Selection.Merge(CellRef.Parse("B2"));
+
+        Assert.Equal("A1:C2", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SelectionMovingTests.cs

@@ -0,0 +1,155 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SelectionMovingTests
+{
+    readonly Sheet sheet = new(4, 4);
+
+    [Fact]
+    public void Should_MoveToTheNextHorizontalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(0, 1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_NotMoveToNextHorizontalCellWhenAlreadyAtLastColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("D1"));
+
+        var cell = sheet.Selection.Move(0, 1);
+
+        Assert.Equal("D1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToTheNextVerticalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(1, 0);
+
+        Assert.Equal("A2", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_NotMoveToNextVerticalCellWhenAlreadyAtLastRow()
+    {
+        sheet.Selection.Select(CellRef.Parse("A4"));
+
+        var cell = sheet.Selection.Move(1, 0);
+
+        Assert.Equal("A4", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToThePreviousHorizontalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        var cell = sheet.Selection.Move(0, -1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_NotMoveToPreviousHorizontalCellWhenAlreadyAtFirstColumn()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(0, -1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToThePreviousVerticalCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("A2"));
+
+        var cell = sheet.Selection.Move(-1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_NotMoveToPreviousVerticalCellWhenAlreadyAtFirstRow()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(-1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToNextHorizontalCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A1:B1"));
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(0, 1);
+
+        Assert.Equal("C1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToNextVerticalCellAfterMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A1:A2"));
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        var cell = sheet.Selection.Move(1, 0);
+
+        Assert.Equal("A3", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToPreviousHorizontalCellBeforeMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        var cell = sheet.Selection.Move(0, -1);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveToPreviousVerticalCellBeforeMergedCell()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A2:A3"));
+        sheet.Selection.Select(CellRef.Parse("A2"));
+
+        var cell = sheet.Selection.Move(-1, 0);
+
+        Assert.Equal("A1", cell.ToString());
+        Assert.Equal(sheet.Selection.Cell, cell);
+    }
+
+    [Fact]
+    public void Should_MoveTheSelectedCellOnly()
+    {
+        sheet.Selection.Select(RangeRef.Parse("A1:C1"));
+
+        var cell = sheet.Selection.Move(0, 1);
+
+        Assert.Equal("B1", cell.ToString());
+        Assert.Equal("B1:B1", sheet.Selection.Range.ToString());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SelectionTests.cs

@@ -0,0 +1,75 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SelectionTests
+{
+    readonly Sheet sheet = new(4, 4);
+
+    [Fact]
+    public void Should_SelectSingleCell()
+    {
+        sheet.Selection.Select(CellRef.Parse("A1"));
+
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+        Assert.Equal("A1:A1", sheet.Selection.Range.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectRangeOfCells()
+    {
+        sheet.Selection.Select(RangeRef.Parse("A1:C3"));
+
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+        Assert.Equal("A1", sheet.Selection.Range.Start.ToString());
+        Assert.Equal("C3", sheet.Selection.Range.End.ToString());
+        Assert.Equal("A1:C3", sheet.Selection.Range.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectTheWholeMergedCellAsPartOfRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(RangeRef.Parse("A1:B1"));
+
+        Assert.Equal("A1:C1", sheet.Selection.Range.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectTheWholeMergedCellByStart()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("B1"));
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectTheWholeMergedCellByEnd()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("C1"));
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectTheWholeMergedCellByEndAndRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("B1:C1"));
+        sheet.Selection.Select(CellRef.Parse("C1"), RangeRef.Parse("B1:C1"));
+
+        Assert.Equal("B1:C1", sheet.Selection.Range.ToString());
+        Assert.Equal("B1", sheet.Selection.Cell.ToString());
+    }
+
+    [Fact]
+    public void Should_SelectCompleteMergedCellRange()
+    {
+        sheet.MergedCells.Add(RangeRef.Parse("A1:D1"));
+        sheet.Selection.Select(RangeRef.Parse("A1:B2"));
+
+        Assert.Equal("A1:D2", sheet.Selection.Range.ToString());
+        Assert.Equal("A1", sheet.Selection.Cell.ToString());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SheetRangeTests.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Linq;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SheetRangeTests
+{
+    [Fact]
+    public void Range_Parse_ShouldParseValidRange()
+    {
+        var range = RangeRef.Parse("A1:B2");
+        
+        Assert.Equal(new CellRef(0, 0), range.Start);
+        Assert.Equal(new CellRef(1, 1), range.End);
+    }
+
+    [Fact]
+    public void Range_Parse_ShouldThrowOnInvalidRange()
+    {
+        Assert.Throws<ArgumentException>(() => RangeRef.Parse("A1:B2:C3"));
+    }
+
+    [Fact]
+    public void Range_GetCells_ShouldReturnAllCellsInRange()
+    {
+        var range = RangeRef.Parse("A1:B2");
+        var cells = range.GetCells().ToList();
+        
+        Assert.Equal(4, cells.Count);
+        Assert.Equal(new CellRef(0, 0), cells[0]); // A1
+        Assert.Equal(new CellRef(0, 1), cells[1]); // B1
+        Assert.Equal(new CellRef(1, 0), cells[2]); // A2
+        Assert.Equal(new CellRef(1, 1), cells[3]); // B2
+    }
+
+    [Fact]
+    public void Range_ToString_ShouldReturnA1Notation()
+    {
+        var range = new RangeRef(new CellRef(0, 0), new CellRef(1, 1));
+        Assert.Equal("A1:B2", range.ToString());
+    }
+} 

Radzen.Blazor.Tests/Spreadsheet/SmallFunctionTests.cs

@@ -0,0 +1,58 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SmallFunctionTests
+{
+    readonly Sheet sheet = new(12, 12);
+
+    [Fact]
+    public void ShouldReturn4thSmallestInFirstColumn()
+    {
+        sheet.Cells["A2"].Value = 3;
+        sheet.Cells["A3"].Value = 4;
+        sheet.Cells["A4"].Value = 5;
+        sheet.Cells["A5"].Value = 2;
+        sheet.Cells["A6"].Value = 3;
+        sheet.Cells["A7"].Value = 4;
+        sheet.Cells["A8"].Value = 6;
+        sheet.Cells["A9"].Value = 4;
+        sheet.Cells["A10"].Value = 7;
+
+        sheet.Cells["B1"].Formula = "=SMALL(A2:A10,4)";
+
+        Assert.Equal(4d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturn2ndSmallestInSecondColumn()
+    {
+        sheet.Cells["B2"].Value = 1;
+        sheet.Cells["B3"].Value = 4;
+        sheet.Cells["B4"].Value = 8;
+        sheet.Cells["B5"].Value = 3;
+        sheet.Cells["B6"].Value = 7;
+        sheet.Cells["B7"].Value = 12;
+        sheet.Cells["B8"].Value = 54;
+        sheet.Cells["B9"].Value = 8;
+        sheet.Cells["B10"].Value = 23;
+
+        sheet.Cells["C1"].Formula = "=SMALL(B2:B10,2)";
+
+        Assert.Equal(3d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldErrorForEmptyArrayOrInvalidK()
+    {
+        sheet.Cells["A1"].Formula = "=SMALL(A2:A2,1)"; // empty range
+        Assert.Equal(CellError.Num, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Value = 5;
+        sheet.Cells["A3"].Formula = "=SMALL(A2:A2,0)"; // k <= 0
+        Assert.Equal(CellError.Num, sheet.Cells["A3"].Value);
+
+        sheet.Cells["A4"].Formula = "=SMALL(A2:A2,2)"; // k > count
+        Assert.Equal(CellError.Num, sheet.Cells["A4"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SortCommandTests.cs

@@ -0,0 +1,147 @@
+using System;
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SortCommandTests
+{
+    [Fact]
+    public void SortCommand_ShouldSortDataInAscendingOrder()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var range = RangeRef.Parse("A1:A3");
+        
+        // Set up test data
+        sheet.Cells["A1"].Value = "Charlie";
+        sheet.Cells["A2"].Value = "Alice";
+        sheet.Cells["A3"].Value = "Bob";
+        
+        var command = new SortCommand(sheet, range, SortOrder.Ascending, 0);
+        
+        // Act
+        var result = command.Execute();
+        
+        // Assert
+        Assert.True(result);
+        Assert.Equal("Alice", sheet.Cells["A1"].Value);
+        Assert.Equal("Bob", sheet.Cells["A2"].Value);
+        Assert.Equal("Charlie", sheet.Cells["A3"].Value);
+    }
+    
+    [Fact]
+    public void SortCommand_ShouldSortDataInDescendingOrder()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var range = RangeRef.Parse("A1:A3");
+        
+        // Set up test data
+        sheet.Cells["A1"].Value = "Alice";
+        sheet.Cells["A2"].Value = "Bob";
+        sheet.Cells["A3"].Value = "Charlie";
+        
+        var command = new SortCommand(sheet, range, SortOrder.Descending, 0);
+        
+        // Act
+        var result = command.Execute();
+        
+        // Assert
+        Assert.True(result);
+        Assert.Equal("Charlie", sheet.Cells["A1"].Value);
+        Assert.Equal("Bob", sheet.Cells["A2"].Value);
+        Assert.Equal("Alice", sheet.Cells["A3"].Value);
+    }
+    
+    [Fact]
+    public void SortCommand_ShouldRestoreOriginalOrderWhenUndone()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var range = RangeRef.Parse("A1:A3");
+        
+        // Set up test data
+        sheet.Cells["A1"].Value = "Charlie";
+        sheet.Cells["A2"].Value = "Alice";
+        sheet.Cells["A3"].Value = "Bob";
+        
+        var command = new SortCommand(sheet, range, SortOrder.Ascending, 0);
+        
+        // Act
+        command.Execute();
+        command.Unexecute();
+        
+        // Assert
+        Assert.Equal("Charlie", sheet.Cells["A1"].Value);
+        Assert.Equal("Alice", sheet.Cells["A2"].Value);
+        Assert.Equal("Bob", sheet.Cells["A3"].Value);
+    }
+    
+    [Fact]
+    public void SortCommand_ShouldReturnFalseForInvalidRange()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var command = new SortCommand(sheet, RangeRef.Invalid, SortOrder.Ascending, 0);
+        
+        // Act
+        var result = command.Execute();
+        
+        // Assert
+        Assert.False(result);
+    }
+    
+    [Fact]
+    public void SortCommand_ShouldPreserveCellFormatting()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var range = RangeRef.Parse("A1:A2");
+        
+        // Set up test data with formatting
+        sheet.Cells["A1"].Value = "Charlie";
+        sheet.Cells["A1"].Format.Bold = true;
+        sheet.Cells["A2"].Value = "Alice";
+        sheet.Cells["A2"].Format.Italic = true;
+        
+        var command = new SortCommand(sheet, range, SortOrder.Ascending, 0);
+        
+        // Act
+        command.Execute();
+        command.Unexecute();
+        
+        // Assert
+        Assert.Equal("Charlie", sheet.Cells["A1"].Value);
+        Assert.True(sheet.Cells["A1"].Format.Bold);
+        Assert.Equal("Alice", sheet.Cells["A2"].Value);
+        Assert.True(sheet.Cells["A2"].Format.Italic);
+    }
+
+    [Fact]
+    public void SortCommand_ShouldWorkWithAutoFilterRange()
+    {
+        // Arrange
+        var sheet = new Sheet(5, 5);
+        var range = RangeRef.Parse("A1:B3");
+        
+        // Set up test data in a format similar to AutoFilter
+        sheet.Cells["A1"].Value = "Name";
+        sheet.Cells["B1"].Value = "Age";
+        sheet.Cells["A2"].Value = "Charlie";
+        sheet.Cells["B2"].Value = 30;
+        sheet.Cells["A3"].Value = "Alice";
+        sheet.Cells["B3"].Value = 25;
+        
+        var command = new SortCommand(sheet, range, SortOrder.Ascending, 0, skipHeaderRow: true);
+        
+        // Act
+        var result = command.Execute();
+        
+        // Assert
+        Assert.True(result);
+        Assert.Equal("Alice", sheet.Cells["A2"].Value);
+        Assert.Equal(25.0, sheet.Cells["B2"].Value);
+        Assert.Equal("Charlie", sheet.Cells["A3"].Value);
+        Assert.Equal(30.0, sheet.Cells["B3"].Value);
+    }
+} 

Radzen.Blazor.Tests/Spreadsheet/SortingTests.cs

@@ -0,0 +1,231 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SortingTests
+{
+    private readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void Should_SortStringsInAscendingOrder()
+    {
+        sheet.Cells[0, 0].Value = "C";
+        sheet.Cells[1, 0].Value = "A";
+        sheet.Cells[2, 0].Value = "B";
+
+        sheet.Sort(RangeRef.Parse("A1:A3"), SortOrder.Ascending);
+
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("B", sheet.Cells[1, 0].Value);
+        Assert.Equal("C", sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void Should_SortStringsInDescendingOrder()
+    {
+        sheet.Cells[0, 0].Value = "C";
+        sheet.Cells[1, 0].Value = "A";
+        sheet.Cells[2, 0].Value = "B";
+
+        sheet.Sort(RangeRef.Parse("A1:A3"), SortOrder.Descending);
+
+        Assert.Equal("C", sheet.Cells[0, 0].Value);
+        Assert.Equal("B", sheet.Cells[1, 0].Value);
+        Assert.Equal("A", sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void Should_SortNumbersInDescendingOrder()
+    {
+        sheet.Cells[0, 0].Value = 1;
+        sheet.Cells[1, 0].Value = 3;
+        sheet.Cells[2, 0].Value = 2;
+
+        sheet.Sort(RangeRef.Parse("A1:A3"), SortOrder.Descending);
+
+        Assert.Equal(3d, sheet.Cells[0, 0].Value);
+        Assert.Equal(2d, sheet.Cells[1, 0].Value);
+        Assert.Equal(1d, sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void Should_SortFullRowsByColumnA()
+    {
+        sheet.Cells[0, 0].Value = "B"; sheet.Cells[0, 1].Value = "B2";
+        sheet.Cells[1, 0].Value = "A"; sheet.Cells[1, 1].Value = "A2";
+        sheet.Cells[2, 0].Value = "C"; sheet.Cells[2, 1].Value = "C2";
+
+        sheet.Sort(RangeRef.Parse("A1:B3"), SortOrder.Ascending);
+
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("A2", sheet.Cells[0, 1].Value);
+        Assert.Equal("B", sheet.Cells[1, 0].Value);
+        Assert.Equal("B2", sheet.Cells[1, 1].Value);
+        Assert.Equal("C", sheet.Cells[2, 0].Value);
+        Assert.Equal("C2", sheet.Cells[2, 1].Value);
+    }
+
+    [Fact]
+    public void Should_IgnoreHeaderRowInSort()
+    {
+        sheet.Cells[0, 0].Value = "Header";
+        sheet.Cells[1, 0].Value = "C";
+        sheet.Cells[2, 0].Value = "A";
+        sheet.Cells[3, 0].Value = "B";
+
+        sheet.Sort(RangeRef.Parse("A2:A4"), SortOrder.Ascending);
+
+        Assert.Equal("Header", sheet.Cells[0, 0].Value); // header unchanged
+        Assert.Equal("A", sheet.Cells[1, 0].Value);
+        Assert.Equal("B", sheet.Cells[2, 0].Value);
+        Assert.Equal("C", sheet.Cells[3, 0].Value);
+    }
+
+    [Fact]
+    public void Should_SortOnlySelectedRange_NotWholeSheet()
+    {
+        sheet.Cells[0, 0].Value = "Z";
+        sheet.Cells[1, 0].Value = "X";
+        sheet.Cells[2, 0].Value = "Y";
+        sheet.Cells[3, 0].Value = "A";
+
+        sheet.Sort(RangeRef.Parse("A2:A4"), SortOrder.Ascending);
+
+        Assert.Equal("Z", sheet.Cells[0, 0].Value); // unchanged
+        Assert.Equal("A", sheet.Cells[1, 0].Value);
+        Assert.Equal("X", sheet.Cells[2, 0].Value);
+        Assert.Equal("Y", sheet.Cells[3, 0].Value);
+    }
+
+    [Fact]
+    public void Should_SortBlanksLastInAscendingOrder()
+    {
+        sheet.Cells[0, 0].Value = "B";
+        sheet.Cells[1, 0].Value = null; // blank
+        sheet.Cells[2, 0].Value = "A";
+
+        sheet.Sort(RangeRef.Parse("A1:A3"), SortOrder.Ascending);
+
+        Assert.Equal("A", sheet.Cells[0, 0].Value);
+        Assert.Equal("B", sheet.Cells[1, 0].Value);
+        Assert.Null(sheet.Cells[2, 0].Value);
+    }
+
+    [Fact]
+    public void Should_NotBreakDataAlignmentAcrossColumns()
+    {
+        sheet.Cells[0, 0].Value = "Charlie"; sheet.Cells[0, 1].Value = "30";
+        sheet.Cells[1, 0].Value = "Alice"; sheet.Cells[1, 1].Value = "25";
+        sheet.Cells[2, 0].Value = "Bob"; sheet.Cells[2, 1].Value = "20";
+
+        sheet.Sort(RangeRef.Parse("A1:B3"), SortOrder.Ascending);
+
+        Assert.Equal("Alice", sheet.Cells[0, 0].Value);
+        Assert.Equal(25d, sheet.Cells[0, 1].Value);
+        Assert.Equal("Bob", sheet.Cells[1, 0].Value);
+        Assert.Equal(20d, sheet.Cells[1, 1].Value);
+        Assert.Equal("Charlie", sheet.Cells[2, 0].Value);
+        Assert.Equal(30d, sheet.Cells[2, 1].Value);
+    }
+
+
+    [Fact]
+    public void Should_SortByAbsoluteColumnIndex()
+    {
+        // Data layout now starts at column B (index 1):
+        // | B (Name) | C (Age) | D (Dept) |
+        // |----------|---------|----------|
+        // | Charlie  | 42      | Dev      |
+        // | Alice    | 25      | HR       |
+        // | Bob      | 30      | Sales    |
+
+        sheet.Cells[0, 1].Value = "Charlie"; // B1
+        sheet.Cells[0, 2].Value = 42;        // C1
+        sheet.Cells[0, 3].Value = "Dev";     // D1
+
+        sheet.Cells[1, 1].Value = "Alice";   // B2
+        sheet.Cells[1, 2].Value = 25;        // C2
+        sheet.Cells[1, 3].Value = "HR";      // D2
+
+        sheet.Cells[2, 1].Value = "Bob";     // B3
+        sheet.Cells[2, 2].Value = 30;        // C3
+        sheet.Cells[2, 3].Value = "Sales";   // D3
+
+        // Sort range B1:D3 by **column C** (absolute index 2, "Age")
+        sheet.Sort(RangeRef.Parse("B1:D3"), SortOrder.Ascending, keyIndex: 2);
+
+        // Sorted order: Alice (25), Bob (30), Charlie (42)
+
+        Assert.Equal("Alice", sheet.Cells[0, 1].Value);
+        Assert.Equal(25d, sheet.Cells[0, 2].Value);
+        Assert.Equal("HR", sheet.Cells[0, 3].Value);
+
+        Assert.Equal("Bob", sheet.Cells[1, 1].Value);
+        Assert.Equal(30d, sheet.Cells[1, 2].Value);
+        Assert.Equal("Sales", sheet.Cells[1, 3].Value);
+
+        Assert.Equal("Charlie", sheet.Cells[2, 1].Value);
+        Assert.Equal(42d, sheet.Cells[2, 2].Value);
+        Assert.Equal("Dev", sheet.Cells[2, 3].Value);
+    }
+
+    [Fact]
+    public void Should_SortCellsWithRelativeFormulasCorrectly()
+    {
+        // Initial setup:
+        // A1:  2
+        // A2:  =A1 + 1   (should be 3)
+        // A3:  1
+
+        sheet.Cells[0, 0].Value = 2;           // A1
+        sheet.Cells[1, 0].Formula = "=A1 + 1"; // A2
+        sheet.Cells[2, 0].Value = 1;           // A3
+
+        // Confirm initial formula result
+        Assert.Equal(3d, sheet.Cells[1, 0].Value);
+
+        // Now sort A1:A3 in ascending order
+        sheet.Sort(RangeRef.Parse("A1:A3"), SortOrder.Ascending);
+
+        // The sorted order should be:
+        // A1:  1         (was A3)
+        // A2:  2         (was A1)
+        // A3:  =A1 + 1   (was A2)
+
+        Assert.Equal(1d, sheet.Cells[0, 0].Value);               // A1
+        Assert.Equal(2d, sheet.Cells[1, 0].Value);               // A2
+        Assert.Equal("=A1 + 1", sheet.Cells[2, 0].Formula);     // A3
+        Assert.Equal(2d, sheet.Cells[2, 0].Value);               // A3 evaluated: =1 + 1
+    }
+
+    [Fact]
+    public void Should_PlaceBlankValuesAtBottomInAscendingAndDescending()
+    {
+        sheet.Cells[0, 0].Value = 3;
+        sheet.Cells[1, 0].Value = null; // blank
+        sheet.Cells[2, 0].Value = 1;
+        sheet.Cells[3, 0].Value = 2;
+
+        // Sort ascending
+        sheet.Sort(RangeRef.Parse("A1:A4"), SortOrder.Ascending);
+
+        Assert.Equal(1d, sheet.Cells[0, 0].Value);
+        Assert.Equal(2d, sheet.Cells[1, 0].Value);
+        Assert.Equal(3d, sheet.Cells[2, 0].Value);
+        Assert.Null(sheet.Cells[3, 0].Value);
+
+        // Re-set original order
+        sheet.Cells[0, 0].Value = 3;
+        sheet.Cells[1, 0].Value = null;
+        sheet.Cells[2, 0].Value = 1;
+        sheet.Cells[3, 0].Value = 2;
+
+        // Sort descending
+        sheet.Sort(RangeRef.Parse("A1:A4"), SortOrder.Descending);
+
+        Assert.Equal(3d, sheet.Cells[0, 0].Value);
+        Assert.Equal(2d, sheet.Cells[1, 0].Value);
+        Assert.Equal(1d, sheet.Cells[2, 0].Value);
+        Assert.Null(sheet.Cells[3, 0].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SpreadsheetClipboardTests.cs

@@ -0,0 +1,60 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SpreadsheetClipboardTests
+{
+    [Fact]
+    public void Copy_Down_AdjustsRelative_KeepsAbsolute()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells[0, 0].Formula = "=A1";        // A1
+        sheet.PasteRange(sheet, RangeRef.Parse("A1"), CellRef.Parse("A2"), FormulaAdjustment.AdjustRelative);
+        Assert.Equal("=A2", sheet.Cells[1, 0].Formula);
+
+        sheet.Cells[0, 1].Formula = "=$A$1";     // B1
+        sheet.PasteRange(sheet, RangeRef.Parse("B1"), CellRef.Parse("B2"), FormulaAdjustment.AdjustRelative);
+        Assert.Equal("=$A$1", sheet.Cells[1, 1].Formula);
+    }
+
+    [Fact]
+    public void Copy_Right_AdjustsColumn_RetainsAbsoluteColumn()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells[0, 0].Formula = "=A1"; // A1
+        sheet.PasteRange(sheet, RangeRef.Parse("A1"), CellRef.Parse("B1"), FormulaAdjustment.AdjustRelative);
+        Assert.Equal("=B1", sheet.Cells[0, 1].Formula);
+
+        sheet.Cells[0, 1].Formula = "=$A1"; // B1 absolute column
+        sheet.PasteRange(sheet, RangeRef.Parse("B1"), CellRef.Parse("C1"), FormulaAdjustment.AdjustRelative);
+        Assert.Equal("=$A1", sheet.Cells[0, 2].Formula);
+    }
+
+    [Fact]
+    public void Cut_DoesNotAdjustFormula()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells[0, 0].Formula = "=A1"; // A1
+        sheet.Selection.Select(CellRef.Parse("A1"));
+        var clipboard = new SpreadsheetClipboard();
+        clipboard.Cut(sheet);
+        clipboard.Paste(sheet, CellRef.Parse("B2"));
+        Assert.Equal("=A1", sheet.Cells[1, 1].Formula); // not adjusted
+        Assert.Null(sheet.Cells[0, 0].Formula);          // source cleared
+        Assert.Null(sheet.Cells[0, 0].Value);
+    }
+
+    [Fact]
+    public void Copy_AcrossSheets_AdjustsRelativeReferences()
+    {
+        var source = new Sheet(10, 10);
+        var target = new Sheet(10, 10);
+
+        source.Cells[0, 0].Formula = "=A1";
+
+        // Copy A1 from source to B2 in target
+        target.PasteRange(source, RangeRef.Parse("A1"), CellRef.Parse("B2"), FormulaAdjustment.AdjustRelative);
+
+        Assert.Equal("=B2", target.Cells[1, 1].Formula);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SubstituteFunctionTests.cs

@@ -0,0 +1,33 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SubstituteFunctionTests
+{
+    [Fact]
+    public void Substitute_AllOccurrences()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "Sales Data";
+        sheet.Cells["B1"].Formula = "=SUBSTITUTE(A2, \"Sales\", \"Cost\")";
+        Assert.Equal("Cost Data", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Substitute_FirstInstanceOnly()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "Quarter 1, 2008";
+        sheet.Cells["B1"].Formula = "=SUBSTITUTE(A3, \"1\", \"2\", 1)";
+        Assert.Equal("Quarter 2, 2008", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Substitute_ThirdInstanceOnly()
+    {
+        var sheet = new Sheet(10, 20);
+        sheet.Cells["A4"].Value = "Quarter 1, 2011";
+        sheet.Cells["B1"].Formula = "=SUBSTITUTE(A4, \"1\", \"2\", 3)";
+        Assert.Equal("Quarter 1, 2012", sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SubtotalFunctionTests.cs

@@ -0,0 +1,94 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SubtotalFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldSumWithCode9()
+    {
+        sheet.Cells["A2"].Value = 120;
+        sheet.Cells["A3"].Value = 10;
+        sheet.Cells["A4"].Value = 150;
+        sheet.Cells["A5"].Value = 23;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(9,A2:A5)";
+
+        Assert.Equal(303d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldAverageWithCode1()
+    {
+        sheet.Cells["A2"].Value = 120;
+        sheet.Cells["A3"].Value = 10;
+        sheet.Cells["A4"].Value = 150;
+        sheet.Cells["A5"].Value = 23;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(1,A2:A5)";
+
+        Assert.Equal(75.75, sheet.Cells["B1"].Value);
+    }
+    [Fact]
+    public void ShouldCountWithCode2()
+    {
+        sheet.Cells["A2"].Value = 120;
+        sheet.Cells["A3"].Value = "x"; // non-numeric, ignored by COUNT
+        sheet.Cells["A4"].Value = 150;
+        sheet.Cells["A5"].Value = null;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(2,A2:A5)";
+        Assert.Equal(2d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldCountAWithCode3()
+    {
+        sheet.Cells["A2"].Value = 120;
+        sheet.Cells["A3"].Value = "x";
+        sheet.Cells["A4"].Value = null;
+        sheet.Cells["A5"].Value = 23;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(3,A2:A5)";
+        Assert.Equal(3d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldMaxWithCode4()
+    {
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Value = 40;
+        sheet.Cells["A4"].Value = 30;
+        sheet.Cells["A5"].Value = 20;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(4,A2:A5)";
+        Assert.Equal(40d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldMinWithCode5()
+    {
+        sheet.Cells["A2"].Value = 10;
+        sheet.Cells["A3"].Value = 40;
+        sheet.Cells["A4"].Value = 30;
+        sheet.Cells["A5"].Value = 20;
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(5,A2:A5)";
+        Assert.Equal(10d, sheet.Cells["B1"].Value);
+    }
+
+    [Fact]
+    public void ShouldRespectHiddenRowsWith109()
+    {
+        sheet.Cells["A2"].Value = 120;
+        sheet.Cells["A3"].Value = 10;
+        sheet.Cells["A4"].Value = 150;
+        sheet.Cells["A5"].Value = 20;
+        sheet.Rows.Hide(2); // hide row 3 (A3)
+
+        sheet.Cells["B1"].Formula = "=SUBTOTAL(109,A2:A5)";
+        Assert.Equal(290d, sheet.Cells["B1"].Value); // 120 + 150 + 20 (excludes hidden 10)
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/SumFunctionTests.cs

@@ -0,0 +1,73 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SumFunctionTests
+{
+    readonly Sheet sheet = new(5, 5);
+
+    [Fact]
+    public void ShouldEvaluateSumFunction()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["A3"].Formula = "=SUM(A1,A2)";
+
+        Assert.Equal(3d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumFunctionWithEmptyCells()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A3"].Formula = "=SUM(A1,A2)";
+
+        Assert.Equal(1d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumFunctionWithMultipleArguments()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["A3"].Value = 3;
+        sheet.Cells["A4"].Formula = "=SUM(A1,A2,A3)";
+
+        Assert.Equal(6d, sheet.Cells["A4"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForEmptySumFunction()
+    {
+        sheet.Cells["A1"].Formula = "=SUM()";
+
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSumRangeOfCells()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["A3"].Formula = "=SUM(A1:A2)";
+
+        Assert.Equal(3d, sheet.Cells["A3"].Value);
+    }
+
+    [Fact]
+    public void ShouldSumNumbersOfDifferentTypes()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2.5;
+        sheet.Cells["A3"].Formula = "=SUM(A1,A2)";
+
+        Assert.Equal(3.5, sheet.Cells["A3"].Value);
+
+        sheet.Cells["A4"].Value = 2.5;
+        sheet.Cells["A5"].Formula = "=SUM(A4,A1)";
+
+        Assert.Equal(3.5, sheet.Cells["A5"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/SumIfFunctionTests.cs

@@ -0,0 +1,278 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class SumIfFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithNumericCriteria()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = 100000;
+        sheet.Cells["A2"].Value = 200000;
+        sheet.Cells["A3"].Value = 300000;
+        sheet.Cells["A4"].Value = 400000;
+        
+        sheet.Cells["B1"].Value = 7000;
+        sheet.Cells["B2"].Value = 14000;
+        sheet.Cells["B3"].Value = 21000;
+        sheet.Cells["B4"].Value = 28000;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\">160000\",B1:B4)";
+
+        Assert.Equal(63000d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithoutSumRange()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = 100000;
+        sheet.Cells["A2"].Value = 200000;
+        sheet.Cells["A3"].Value = 300000;
+        sheet.Cells["A4"].Value = 400000;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\">160000\")";
+
+        Assert.Equal(900000d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithExactMatch()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = 100000;
+        sheet.Cells["A2"].Value = 200000;
+        sheet.Cells["A3"].Value = 300000;
+        sheet.Cells["A4"].Value = 400000;
+        
+        sheet.Cells["B1"].Value = 7000;
+        sheet.Cells["B2"].Value = 14000;
+        sheet.Cells["B3"].Value = 21000;
+        sheet.Cells["B4"].Value = 28000;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,300000,B1:B4)";
+
+        Assert.Equal(21000d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithStringCriteria()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = "Vegetables";
+        sheet.Cells["A2"].Value = "Vegetables";
+        sheet.Cells["A3"].Value = "Fruits";
+        sheet.Cells["A4"].Value = "";
+        sheet.Cells["A5"].Value = "Vegetables";
+        sheet.Cells["A6"].Value = "Fruits";
+        
+        sheet.Cells["C1"].Value = 2300;
+        sheet.Cells["C2"].Value = 5500;
+        sheet.Cells["C3"].Value = 800;
+        sheet.Cells["C4"].Value = 400;
+        sheet.Cells["C5"].Value = 4200;
+        sheet.Cells["C6"].Value = 1200;
+
+        sheet.Cells["D1"].Formula = "=SUMIF(A1:A6,\"Fruits\",C1:C6)";
+
+        Assert.Equal(2000d, sheet.Cells["D1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithWildcardPattern()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = "Vegetables";
+        sheet.Cells["A2"].Value = "Vegetables";
+        sheet.Cells["A3"].Value = "Fruits";
+        sheet.Cells["A4"].Value = "";
+        sheet.Cells["A5"].Value = "Vegetables";
+        sheet.Cells["A6"].Value = "Fruits";
+        
+        sheet.Cells["B1"].Value = "Tomatoes";
+        sheet.Cells["B2"].Value = "Celery";
+        sheet.Cells["B3"].Value = "Oranges";
+        sheet.Cells["B4"].Value = "Butter";
+        sheet.Cells["B5"].Value = "Carrots";
+        sheet.Cells["B6"].Value = "Apples";
+        
+        sheet.Cells["C1"].Value = 2300;
+        sheet.Cells["C2"].Value = 5500;
+        sheet.Cells["C3"].Value = 800;
+        sheet.Cells["C4"].Value = 400;
+        sheet.Cells["C5"].Value = 4200;
+        sheet.Cells["C6"].Value = 1200;
+
+        sheet.Cells["D1"].Formula = "=SUMIF(B1:B6,\"*es\",C1:C6)";
+
+        Assert.Equal(4300d, sheet.Cells["D1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithEmptyCriteria()
+    {
+        // Test data from Excel example
+        sheet.Cells["A1"].Value = "Vegetables";
+        sheet.Cells["A2"].Value = "Vegetables";
+        sheet.Cells["A3"].Value = "Fruits";
+        sheet.Cells["A4"].Value = "";
+        sheet.Cells["A5"].Value = "Vegetables";
+        sheet.Cells["A6"].Value = "Fruits";
+        
+        sheet.Cells["C1"].Value = 2300;
+        sheet.Cells["C2"].Value = 5500;
+        sheet.Cells["C3"].Value = 800;
+        sheet.Cells["C4"].Value = 400;
+        sheet.Cells["C5"].Value = 4200;
+        sheet.Cells["C6"].Value = 1200;
+
+        sheet.Cells["D1"].Formula = "=SUMIF(A1:A6,\"\",C1:C6)";
+
+        Assert.Equal(400d, sheet.Cells["D1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithLessThanCriteria()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["A4"].Value = 40;
+        
+        sheet.Cells["B1"].Value = 100;
+        sheet.Cells["B2"].Value = 200;
+        sheet.Cells["B3"].Value = 300;
+        sheet.Cells["B4"].Value = 400;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\"<25\",B1:B4)";
+
+        Assert.Equal(300d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithGreaterThanOrEqualCriteria()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["A4"].Value = 40;
+        
+        sheet.Cells["B1"].Value = 100;
+        sheet.Cells["B2"].Value = 200;
+        sheet.Cells["B3"].Value = 300;
+        sheet.Cells["B4"].Value = 400;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\">=25\",B1:B4)";
+
+        Assert.Equal(700d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithNotEqualCriteria()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["A4"].Value = 40;
+        
+        sheet.Cells["B1"].Value = 100;
+        sheet.Cells["B2"].Value = 200;
+        sheet.Cells["B3"].Value = 300;
+        sheet.Cells["B4"].Value = 400;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\"<>20\",B1:B4)";
+
+        Assert.Equal(800d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithQuestionMarkWildcard()
+    {
+        sheet.Cells["A1"].Value = "cat";
+        sheet.Cells["A2"].Value = "bat";
+        sheet.Cells["A3"].Value = "rat";
+        sheet.Cells["A4"].Value = "goat";
+        
+        sheet.Cells["B1"].Value = 10;
+        sheet.Cells["B2"].Value = 20;
+        sheet.Cells["B3"].Value = 30;
+        sheet.Cells["B4"].Value = 40;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A4,\"?at\",B1:B4)";
+
+        Assert.Equal(60d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldEvaluateSumIfFunctionWithEscapedWildcards()
+    {
+        sheet.Cells["A1"].Value = "*";
+        sheet.Cells["A2"].Value = "?";
+        sheet.Cells["A3"].Value = "test";
+        
+        sheet.Cells["B1"].Value = 10;
+        sheet.Cells["B2"].Value = 20;
+        sheet.Cells["B3"].Value = 30;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A3,\"~*\",B1:B3)";
+
+        Assert.Equal(10d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForInvalidArgumentCount()
+    {
+        sheet.Cells["A1"].Formula = "=SUMIF(A2)";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Value);
+
+        sheet.Cells["A2"].Formula = "=SUMIF(A3,A4,A5,A6)";
+        Assert.Equal(CellError.Value, sheet.Cells["A2"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnValueErrorForMismatchedRangeSizes()
+    {
+        sheet.Cells["A1"].Value = 1;
+        sheet.Cells["A2"].Value = 2;
+        sheet.Cells["B1"].Value = 10;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A2,\">0\",B1)";
+
+        Assert.Equal(CellError.Value, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldHandleEmptyCellsInSumRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        
+        // B2 is empty
+        sheet.Cells["B1"].Value = 100;
+        sheet.Cells["B3"].Value = 300;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A3,\">15\",B1:B3)";
+
+        Assert.Equal(300d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldHandleNonNumericValuesInSumRange()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        
+        sheet.Cells["B1"].Value = 100;
+        sheet.Cells["B2"].Value = "text"; // Non-numeric
+        sheet.Cells["B3"].Value = 300;
+
+        sheet.Cells["C1"].Formula = "=SUMIF(A1:A3,\">15\",B1:B3)";
+
+        Assert.Equal(300d, sheet.Cells["C1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/TextFunctionTests.cs

@@ -0,0 +1,53 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class TextFunctionTests
+{
+    [Fact]
+    public void Text_Currency_Thousands_TwoDecimals()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = 1234.567;
+        sheet.Cells["B1"].Formula = "=TEXT(A1,\"$#,##0.00\")";
+        Assert.Equal("$1,234.57", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Text_Date_MMDDYY()
+    {
+        var sheet = new Sheet(10, 10);
+        var dt = new System.DateTime(2012, 3, 14);
+        sheet.Cells["A1"].Value = dt;
+        sheet.Cells["B1"].Formula = "=TEXT(A1,\"MM/DD/YY\")";
+        Assert.Equal("03/14/12", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Text_Time_12h_AMPM()
+    {
+        var sheet = new Sheet(10, 10);
+        var dt = new System.DateTime(2020, 1, 1, 13, 29, 0);
+        sheet.Cells["A1"].Value = dt;
+        sheet.Cells["B1"].Formula = "=TEXT(A1,\"H:MM AM/PM\")";
+        Assert.Equal("1:29 PM", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Text_Percent_OneDecimal()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = 0.285;
+        sheet.Cells["B1"].Formula = "=TEXT(A1,\"0.0%\")";
+        Assert.Equal("28.5%", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Text_LeadingZeros()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Value = 1234;
+        sheet.Cells["B1"].Formula = "=TEXT(A1,\"0000000\")";
+        Assert.Equal("0001234", sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/TextJoinFunctionTests.cs

@@ -0,0 +1,57 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class TextJoinFunctionTests
+{
+    [Fact]
+    public void TextJoin_Literals_IgnoreEmptyTrue()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=TEXTJOIN(\" \",TRUE,\"The\",\"sun\",\"will\",\"come\",\"up\",\"tomorrow.\")";
+        Assert.Equal("The sun will come up tomorrow.", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void TextJoin_Range_CommaSpace_IgnoreEmptyTrue()
+    {
+        var sheet = new Sheet(20, 10);
+        // A2:A8 values
+        sheet.Cells["A2"].Value = "US Dollar";
+        sheet.Cells["A3"].Value = "Australian Dollar";
+        sheet.Cells["A4"].Value = "Chinese Yuan";
+        sheet.Cells["A5"].Value = "Hong Kong Dollar";
+        sheet.Cells["A6"].Value = "Israeli Shekel";
+        sheet.Cells["A7"].Value = "South Korean Won";
+        sheet.Cells["A8"].Value = "Russian Ruble";
+        sheet.Cells["B1"].Formula = "=TEXTJOIN(\", \", TRUE, A2:A8)";
+        var result = sheet.Cells["B1"].Data.GetValueOrDefault<string>();
+        Assert.Equal("US Dollar, Australian Dollar, Chinese Yuan, Hong Kong Dollar, Israeli Shekel, South Korean Won, Russian Ruble", result);
+    }
+
+    [Fact]
+    public void TextJoin_Range2D_CommaSpace_IgnoreEmptyVariants()
+    {
+        var sheet = new Sheet(20, 10);
+        // A2:B8 grid
+        sheet.Cells["A2"].Value = "a1";
+        sheet.Cells["B2"].Value = "b1";
+        sheet.Cells["A3"].Value = "a2";
+        sheet.Cells["B3"].Value = "b2";
+        sheet.Cells["A4"].Value = string.Empty; // empty cell value
+        sheet.Cells["B4"].Value = string.Empty;
+        sheet.Cells["A5"].Value = "a5";
+        sheet.Cells["B5"].Value = "b5";
+        sheet.Cells["A6"].Value = "a6";
+        sheet.Cells["B6"].Value = "b6";
+        sheet.Cells["A7"].Value = "a7";
+        sheet.Cells["B7"].Value = "b7";
+        sheet.Cells["B1"].Formula = "=TEXTJOIN(\", \", TRUE, A2:B7)";
+        Assert.Equal("a1, b1, a2, b2, a5, b5, a6, b6, a7, b7", sheet.Cells["B1"].Data.Value);
+
+        sheet.Cells["C1"].Formula = "=TEXTJOIN(\", \", FALSE, A2:B7)";
+        Assert.Equal("a1, b1, a2, b2, , , a5, b5, a6, b6, a7, b7", sheet.Cells["C1"].Data.Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/TodayFunctionTests.cs

@@ -0,0 +1,26 @@
+using Xunit;
+using Radzen.Blazor.Spreadsheet;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class TodayFunctionTests
+{
+    [Fact]
+    public void Today_ReturnsCurrentDate()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=TODAY()";
+        var dt = sheet.Cells["A1"].Data.GetValueOrDefault<System.DateTime>();
+        Assert.Equal(System.DateTime.Today.Date, dt.Date);
+    }
+
+    [Fact]
+    public void Today_PlusFiveDays()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=TODAY()+5";
+        var serial = sheet.Cells["A1"].Data.GetValueOrDefault<double>();
+        var expected = System.DateTime.Today.AddDays(5).ToNumber();
+        Assert.Equal(expected, serial);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/TrimFunctionTests.cs

@@ -0,0 +1,31 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class TrimFunctionTests
+{
+    [Fact]
+    public void Trim_RemovesLeadingTrailingAndCollapsesInternalSpaces()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=TRIM(\" First  Quarter   Earnings \")";
+        Assert.Equal("First Quarter Earnings", sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Trim_Empty_ReturnsEmpty()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=TRIM(\"   \")";
+        Assert.Equal(string.Empty, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Trim_TextCell_Works()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["B1"].Value = "  Hello   world  ";
+        sheet.Cells["A1"].Formula = "=TRIM(B1)";
+        Assert.Equal("Hello world", sheet.Cells["A1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/TruncFunctionTests.cs

@@ -0,0 +1,31 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class TruncFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldTruncatePositive()
+    {
+        sheet.Cells["A1"].Formula = "=TRUNC(8.9)";
+        Assert.Equal(8d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTruncateNegative()
+    {
+        sheet.Cells["A1"].Formula = "=TRUNC(0-8.9)";
+        Assert.Equal(-8d, sheet.Cells["A1"].Value);
+    }
+
+    [Fact]
+    public void ShouldTruncateLessThanOne()
+    {
+        sheet.Cells["A1"].Formula = "=TRUNC(0.45)";
+        Assert.Equal(0d, sheet.Cells["A1"].Value);
+    }
+}
+
+

Radzen.Blazor.Tests/Spreadsheet/UpperFunctionTests.cs

@@ -0,0 +1,24 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class UpperFunctionTests
+{
+    [Fact]
+    public void Upper_ConvertsToUppercase()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A2"].Value = "total";
+        sheet.Cells["B1"].Formula = "=UPPER(A2)";
+        Assert.Equal("TOTAL", sheet.Cells["B1"].Data.Value);
+    }
+
+    [Fact]
+    public void Upper_AlreadyUppercase()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A3"].Value = "Yield";
+        sheet.Cells["B1"].Formula = "=UPPER(A3)";
+        Assert.Equal("YIELD", sheet.Cells["B1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/ValueFunctionTests.cs

@@ -0,0 +1,22 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class ValueFunctionTests
+{
+    [Fact]
+    public void Value_ParsesCurrency()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=VALUE(\"$1,000\")";
+        Assert.Equal(1000d, sheet.Cells["A1"].Data.Value);
+    }
+
+    [Fact]
+    public void Value_TimeDifferenceFractionOfDay()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=VALUE(\"16:48:00\")-VALUE(\"12:00:00\")";
+        Assert.Equal(0.2d, sheet.Cells["A1"].Data.Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/VerticalLookupFunctionTests.cs

@@ -0,0 +1,61 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class VerticalLookupFunctionTests
+{
+    readonly Sheet sheet = new(10, 10);
+
+    [Fact]
+    public void ShouldFindExactMatchInTwoColumnRange()
+    {
+        sheet.Cells["A1"].Value = "T-Shirt";
+        sheet.Cells["A2"].Value = "Jeans";
+        sheet.Cells["B1"].Value = 19.99;
+        sheet.Cells["B2"].Value = 29.99;
+
+        sheet.Cells["C1"].Formula = "=VLOOKUP(\"T-Shirt\",A1:B2,2,0)";
+
+        Assert.Equal(19.99, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNAWhenNoExactMatch()
+    {
+        sheet.Cells["A1"].Value = "Hat";
+        sheet.Cells["B1"].Value = 9.99;
+
+        sheet.Cells["C1"].Formula = "=VLOOKUP(\"Gloves\",A1:B1,2,0)";
+
+        Assert.Equal(CellError.NA, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldFindApproximateMatchInSortedFirstColumn()
+    {
+        // First column sorted ascending
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+
+        sheet.Cells["B1"].Value = "Low";
+        sheet.Cells["B2"].Value = "Medium";
+        sheet.Cells["B3"].Value = "High";
+
+        // search_key 25 -> should pick row with 20
+        sheet.Cells["C1"].Formula = "=VLOOKUP(25,A1:B3,2,1)";
+
+        Assert.Equal("Medium", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldErrorWhenIndexOutOfRange()
+    {
+        sheet.Cells["A1"].Value = "X";
+        sheet.Cells["B1"].Value = 1;
+
+        sheet.Cells["C1"].Formula = "=VLOOKUP(\"X\",A1:B1,3,0)";
+
+        Assert.Equal(CellError.Ref, sheet.Cells["C1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/WeekdayFunctionTests.cs

@@ -0,0 +1,42 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class WeekdayFunctionTests
+{
+    [Fact]
+    public void Weekday_Default_SundayToSaturday()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2008, 2, 14)); // Thursday
+        sheet.Cells["B1"].Formula = "=WEEKDAY(A1)"; // default 1: Sun=1..Sat=7
+        Assert.Equal(5, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weekday_Type2_MondayToSunday()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2008, 2, 14)); // Thursday
+        sheet.Cells["B1"].Formula = "=WEEKDAY(A1, 2)"; // Mon=1..Sun=7
+        Assert.Equal(4, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weekday_Type3_MondayZero_SundaySix()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2008, 2, 14)); // Thursday
+        sheet.Cells["B1"].Formula = "=WEEKDAY(A1, 3)"; // Mon=0..Sun=6
+        Assert.Equal(3, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weekday_InvalidReturnType_ReturnsNumError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2008, 2, 14));
+        sheet.Cells["B1"].Formula = "=WEEKDAY(A1, 10)"; // invalid
+        Assert.Equal(CellError.Num, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/WeeknumFunctionTests.cs

@@ -0,0 +1,42 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class WeeknumFunctionTests
+{
+    [Fact]
+    public void Weeknum_Default_SundayStart()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2012, 3, 9)); // Excel example
+        sheet.Cells["B1"].Formula = "=WEEKNUM(A1)"; // default 1
+        Assert.Equal(10, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weeknum_Type2_MondayStart()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2012, 3, 9));
+        sheet.Cells["B1"].Formula = "=WEEKNUM(A1, 2)"; // Monday start, System 1
+        Assert.Equal(11, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weeknum_Type21_ISO()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2012, 3, 9));
+        sheet.Cells["B1"].Formula = "=WEEKNUM(A1, 21)"; // ISO
+        Assert.Equal(10, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Weeknum_InvalidReturnType_ReturnsNumError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2012, 3, 9));
+        sheet.Cells["B1"].Formula = "=WEEKNUM(A1, 4)"; // invalid code
+        Assert.Equal(CellError.Num, sheet.Cells["B1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/XLookupFunctionTests.cs

@@ -0,0 +1,114 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class XLookupFunctionTests
+{
+    readonly Sheet sheet = new(20, 10);
+
+    [Fact]
+    public void ShouldFindExactMatchAndReturnFromAnotherColumn()
+    {
+        sheet.Cells["A1"].Value = "P1";
+        sheet.Cells["A2"].Value = "P2";
+        sheet.Cells["B1"].Value = 10;
+        sheet.Cells["B2"].Value = 20;
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(\"P2\",A1:A2,B1:B2)";
+
+        Assert.Equal(20d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnIfNotFoundValue()
+    {
+        sheet.Cells["A1"].Value = "P1";
+        sheet.Cells["B1"].Value = 10;
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(\"P2\",A1:A1,B1:B1,\"Missing\")";
+
+        Assert.Equal("Missing", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSupportWildcardMatch()
+    {
+        sheet.Cells["A1"].Value = "Item-100";
+        sheet.Cells["A2"].Value = "Item-200";
+        sheet.Cells["B1"].Value = "A";
+        sheet.Cells["B2"].Value = "B";
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(\"Item-2*\",A1:A2,B1:B2,\"\",2)";
+
+        Assert.Equal("B", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldFindNextSmallerWhenNotFound()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["B1"].Value = "L";
+        sheet.Cells["B2"].Value = "M";
+        sheet.Cells["B3"].Value = "H";
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(25,A1:A3,B1:B3,\"\",0-1)";
+
+        Assert.Equal("M", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldFindNextLargerWhenNotFound()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["B1"].Value = "L";
+        sheet.Cells["B2"].Value = "M";
+        sheet.Cells["B3"].Value = "H";
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(25,A1:A3,B1:B3,\"\",1)";
+
+        Assert.Equal("H", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSupportReverseSearch()
+    {
+        sheet.Cells["A1"].Value = "A";
+        sheet.Cells["A2"].Value = "A";
+        sheet.Cells["B1"].Value = 1;
+        sheet.Cells["B2"].Value = 2;
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(\"A\",A1:A2,B1:B2,\"\",0,0-1)";
+
+        Assert.Equal(2d, sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldSupportBinarySearchAscending()
+    {
+        sheet.Cells["A1"].Value = 10;
+        sheet.Cells["A2"].Value = 20;
+        sheet.Cells["A3"].Value = 30;
+        sheet.Cells["B1"].Value = "L";
+        sheet.Cells["B2"].Value = "M";
+        sheet.Cells["B3"].Value = "H";
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(20,A1:A3,B1:B3,\"\",0,2)";
+
+        Assert.Equal("M", sheet.Cells["C1"].Value);
+    }
+
+    [Fact]
+    public void ShouldReturnNAWhenNotFoundAndNoIfNotFound()
+    {
+        sheet.Cells["A1"].Value = "A";
+        sheet.Cells["B1"].Value = 1;
+
+        sheet.Cells["C1"].Formula = "=XLOOKUP(\"B\",A1:A1,B1:B1)";
+
+        Assert.Equal(CellError.NA, sheet.Cells["C1"].Value);
+    }
+}

Radzen.Blazor.Tests/Spreadsheet/YearFunctionTests.cs

@@ -0,0 +1,31 @@
+using Xunit;
+
+namespace Radzen.Blazor.Spreadsheet.Tests;
+
+public class YearFunctionTests
+{
+    [Fact]
+    public void Year_FromDateSerial_ReturnsYear()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=YEAR(VALUE(\"2025-05-23\"))";
+        Assert.Equal(2025, sheet.Cells["A1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Year_FromDateValue_ReturnsYear()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Data = CellData.FromDate(new System.DateTime(2023, 7, 5));
+        sheet.Cells["B1"].Formula = "=YEAR(A1)";
+        Assert.Equal(2023, sheet.Cells["B1"].Data.GetValueOrDefault<double>());
+    }
+
+    [Fact]
+    public void Year_InvalidText_ReturnsValueError()
+    {
+        var sheet = new Sheet(10, 10);
+        sheet.Cells["A1"].Formula = "=YEAR(\"abc\")";
+        Assert.Equal(CellError.Value, sheet.Cells["A1"].Data.GetValueOrDefault<CellError>());
+    }
+}

Radzen.Blazor.Tests/SwitchTests.cs

@@ -110,7 +110,7 @@ namespace Radzen.Blazor.Tests
 
             component.SetParametersAndRender(parameters => parameters.Add(p => p.Change, args => { raised = true; newValue = args; }));
 
-            component.Find("input[type=\"checkbox\"]").Click();
+            component.Find("div").Click();
 
             Assert.True(raised);
             Assert.True(object.Equals(value, !(bool)newValue));
@@ -129,7 +129,7 @@ namespace Radzen.Blazor.Tests
 
             component.SetParametersAndRender(parameters => parameters.Add(p => p.ValueChanged, args => { raised = true; newValue = args; }));
 
-            component.Find("input[type=\"checkbox\"]").Click();
+            component.Find("div").Click();
 
             Assert.True(raised);
             Assert.True(object.Equals(value, !(bool)newValue));

Radzen.Blazor/DialogService.cs

@@ -716,42 +716,6 @@ namespace Radzen
             }
         }
 
-        private string? ariaLabel = "Dialog";
-        /// <summary>
-        /// Gets or sets the dialog aria-label text when no title is rendered.
-        /// </summary>
-        /// <value>The dialog aria-label text.</value>
-        public string? AriaLabel
-        {
-            get => ariaLabel;
-            set
-            {
-                if (ariaLabel != value)
-                {
-                    ariaLabel = value;
-                    OnPropertyChanged(nameof(AriaLabel));
-                }
-            }
-        }
-
-        private string closeAriaLabel = "Close dialog";
-        /// <summary>
-        /// Gets or sets the close button aria-label text.
-        /// </summary>
-        /// <value>The close button aria-label text.</value>
-        public string CloseAriaLabel
-        {
-            get => closeAriaLabel;
-            set
-            {
-                if (closeAriaLabel != value)
-                {
-                    closeAriaLabel = value;
-                    OnPropertyChanged(nameof(CloseAriaLabel));
-                }
-            }
-        }
-
         private string? width;
         /// <summary>
         /// Gets or sets the width of the dialog.

Radzen.Blazor/DropDownBase.cs

@@ -254,13 +254,6 @@ namespace Radzen
         [Parameter]
         public string RemoveChipTitle { get; set; } = "Remove";
 
-        /// <summary>
-        /// Gets or sets the clear button aria label text.
-        /// </summary>
-        /// <value>The clear button aria label text.</value>
-        [Parameter]
-        public string ClearAriaLabel { get; set; } = "Clear";
-
         /// <summary>
         /// Gets or sets the search aria label text.
         /// </summary>
@@ -515,7 +508,7 @@ namespace Radzen
         /// <param name="item">The item.</param>
         /// <param name="property">The property.</param>
         /// <returns>System.Object.</returns>
-        public virtual object? GetItemOrValueFromProperty(object? item, string property)
+        public object? GetItemOrValueFromProperty(object? item, string property)
         {
             if (item != null)
             {
@@ -565,18 +558,6 @@ namespace Radzen
             }
         }
 
-        /// <summary>
-        /// Gets the listbox identifier.
-        /// </summary>
-        /// <value>The listbox identifier.</value>
-        protected string ListId
-        {
-            get
-            {
-                return $"{GetId()}-list";
-            }
-        }
-
         /// <summary>
         /// Gets the search identifier.
         /// </summary>
@@ -654,8 +635,6 @@ namespace Radzen
                 await JSRuntime.InvokeVoidAsync("Radzen.focusElement", isFilter ? UniqueID : SearchID);
             }
 
-            isPopupOpen = true;
-
             if (list != null && JSRuntime != null)
             {
                 await JSRuntime.InvokeVoidAsync("Radzen.selectListItem", search, list, selectedIndex);
@@ -672,11 +651,7 @@ namespace Radzen
         /// <param name="shouldSelectOnChange">Should select item on item change with keyboard.</param>
         protected virtual async Task HandleKeyPress(Microsoft.AspNetCore.Components.Web.KeyboardEventArgs args, bool isFilter = false, bool? shouldSelectOnChange = null)
         {
-            ArgumentNullException.ThrowIfNull(args);
-
-            var key = args.Code != null ? args.Code : args.Key;
-
-            if (Disabled || Data == null || args == null || key == null)
+            if (Disabled || Data == null || args == null)
                 return;
 
             List<object> items = Enumerable.Empty<object>().ToList();
@@ -700,6 +675,8 @@ namespace Radzen
                 }
             }
 
+            var key = args.Code != null ? args.Code : args.Key;
+
             if (!args.AltKey && (key == "ArrowDown" || key == "ArrowLeft" || key == "ArrowUp" || key == "ArrowRight"))
             {
                 preventKeydown = true;
@@ -864,15 +841,8 @@ namespace Radzen
             {
                 await JSRuntime.InvokeVoidAsync("Radzen.closePopup", PopupID);
             }
-
-            isPopupOpen = false;
         }
 
-        /// <summary>
-        /// Gets a value indicating whether the popup is open.
-        /// </summary>
-        protected bool isPopupOpen;
-
         int itemIndex;
         string? previousKey;
 
@@ -966,25 +936,6 @@ namespace Radzen
             }
         }
 
-        /// <summary>
-        /// Handles filter input changes (e.g. paste).
-        /// </summary>
-        /// <param name="args">The <see cref="ChangeEventArgs"/> instance containing the event data.</param>
-        protected virtual async Task OnFilterInput(ChangeEventArgs args)
-        {
-            ArgumentNullException.ThrowIfNull(args);
-
-            searchText = $"{args.Value}"; 
-            await SearchTextChanged.InvokeAsync(searchText);
-
-            if (ResetSelectedIndexOnFilter)
-            {
-                selectedIndex = -1;
-            }
-
-            Debounce(DebounceFilter, FilterDelay);
-        }
-
         /// <summary>
         /// Gets the load data arguments.
         /// </summary>
@@ -1318,25 +1269,9 @@ namespace Radzen
 
                 await Change.InvokeAsync(internalValue);
             }
-
             StateHasChanged();
         }
 
-        /// <summary>
-        /// Handles keyboard activation for the select-all action.
-        /// </summary>
-        /// <param name="args">The <see cref="Microsoft.AspNetCore.Components.Web.KeyboardEventArgs"/> instance containing the event data.</param>
-        protected async Task OnSelectAllKeyDown(Microsoft.AspNetCore.Components.Web.KeyboardEventArgs args)
-        {
-            ArgumentNullException.ThrowIfNull(args);
-
-            var key = args.Code != null ? args.Code : args.Key;
-            if (key == "Enter" || key == "Space")
-            {
-                await SelectAll();
-            }
-        }
-
         /// <inheritdoc />
         public override object? GetValue()
         {

Radzen.Blazor/GroupRowRenderEventArgs.cs

@@ -17,12 +17,6 @@ public class GroupRowRenderEventArgs
     /// </summary>
     public Group? Group { get; internal set; }
 
-    /// <summary>
-    /// Gets or sets a value indicating whether this group row is expandable.
-    /// </summary>
-    /// <value><c>true</c> if expandable; otherwise, <c>false</c>.</value>
-    public bool Expandable { get; set; }
-
     /// <summary>
     /// Gets or sets a value indicating whether this group row is expanded.
     /// </summary>

Radzen.Blazor/IRadzenSpiderChart.cs

@@ -1,22 +0,0 @@
-using System.Threading.Tasks;
-
-namespace Radzen.Blazor
-{
-    /// <summary>
-    /// Non-generic contract for <see cref="RadzenSpiderChart"/> used by configuration components
-    /// like <see cref="RadzenSpiderLegend"/> without relying on reflection (important for trimming/AOT).
-    /// </summary>
-    public interface IRadzenSpiderChart
-    {
-        /// <summary>
-        /// Gets or sets the legend configuration for the chart.
-        /// </summary>
-        RadzenSpiderLegend Legend { get; set; }
-
-        /// <summary>
-        /// Requests the chart to refresh its rendering.
-        /// </summary>
-        Task Refresh();
-    }
-}
-

Radzen.Blazor/IRadzenSpiderSeries.cs

@@ -1,29 +0,0 @@
-using Microsoft.AspNetCore.Components;
-using System.Collections.Generic;
-
-namespace Radzen.Blazor
-{
-    /// <summary>
-    /// Non-generic contract for spider series so <see cref="RadzenSpiderChart"/> can be non-generic.
-    /// </summary>
-    internal interface IRadzenSpiderSeries
-    {
-        int Index { get; set; }
-        string Title { get; }
-        bool IsVisible { get; set; }
-        bool MarkersVisible { get; }
-        double MarkerSize { get; }
-        double StrokeWidth { get; }
-
-        IEnumerable<string> GetCategories();
-        IEnumerable<double> GetValues();
-        double GetValue(string category);
-        object? GetData(string category);
-        string FormatValue(double value);
-
-        double MeasureLegend();
-        RenderFragment RenderLegendItem();
-        void ForceUpdate();
-    }
-}
-

Radzen.Blazor/NotificationService.cs

@@ -152,12 +152,6 @@ namespace Radzen
         /// <value>The summary content.</value>
         public RenderFragment<NotificationService>? SummaryContent { get; set; }
 
-        /// <summary>
-        /// Gets or sets the close button aria-label.
-        /// </summary>
-        /// <value>The close button aria-label.</value>
-        public string CloseAriaLabel { get; set; } = "Close";
-
 
         #region Implementation of IEquatable<NotificationMessage> and operators overloading
 

Radzen.Blazor/QueryableExtension.cs

@@ -41,65 +41,6 @@ namespace Radzen
             return source.Provider.CreateQuery(selectExpression);
         }
 
-        /// <summary>
-        /// Projects each element of a sequence into a collection of property values.
-        /// </summary>
-        public static IQueryable Select(this IQueryable source, IEnumerable<string> propertyNames)
-        {
-            ArgumentNullException.ThrowIfNull(source);
-            ArgumentNullException.ThrowIfNull(propertyNames);
-
-            var parameter = Expression.Parameter(source.ElementType, "x");
-
-            var bindings = new List<MemberBinding>();
-            var allProperties = source.ElementType.GetProperties();
-
-            foreach (var property in allProperties.Where(p => propertyNames.Contains(p.Name)))
-            {
-                bindings.Add(Expression.Bind(property, Expression.Property(parameter, property)));
-            }
-
-            var body = Expression.MemberInit(Expression.New(source.ElementType), bindings);
-
-            var delegateType = typeof(Func<,>).MakeGenericType(source.ElementType, source.ElementType);
-
-            var lambda = Expression.Lambda(delegateType, body, parameter);
-
-            var selectExpression = Expression.Call(typeof(Queryable),
-                nameof(Queryable.Select), [source.ElementType, source.ElementType], source.Expression,
-                    Expression.Quote(lambda));
-
-            return source.Provider.CreateQuery(selectExpression);
-        }
-
-        /// <summary>
-        /// Projects each element of a sequence into a collection of property values.
-        /// </summary>
-        public static IQueryable<T> Select<T>(this IQueryable<T> source, IEnumerable<string> propertyNames)
-        {
-            ArgumentNullException.ThrowIfNull(source);
-            ArgumentNullException.ThrowIfNull(propertyNames);
-
-            var parameter = Expression.Parameter(typeof(T), "x");
-
-            var bindings = new List<MemberBinding>();
-            var allProperties = typeof(T).GetProperties();
-
-            foreach (var property in allProperties.Where(p => propertyNames.Contains(p.Name)))
-            {
-                bindings.Add(Expression.Bind(property, Expression.Property(parameter, property)));
-            }
-
-            var body = Expression.MemberInit(Expression.New(typeof(T)), bindings);
-
-            var lambda = Expression.Lambda<Func<T, T>>(body, parameter);
-
-            var selectExpression = Expression.Call(typeof(Queryable),
-                nameof(Queryable.Select), [typeof(T), typeof(T)], source.Expression,
-                    Expression.Quote(lambda));
-
-            return source.Provider.CreateQuery<T>(selectExpression);
-        }
 
         /// <summary>
         /// Projects each element of a sequence to an IEnumerable and flattens the resulting sequences into one sequence.
@@ -1754,7 +1695,7 @@ namespace Radzen
             return columns
                 .Where(c => c.Filterable
                     && c.FilterPropertyType != null
-                    && (!(c.GetFilterValue() == null || (c.GetFilterValue() as string)?.Length == 0)
+                    && (!string.IsNullOrEmpty(c.GetFilterValue() as string)
                         || !c.CanSetFilterValue()
                         || c.HasCustomFilter())
                     && c.GetFilterProperty() != null)

Radzen.Blazor/Radzen.Blazor.csproj

@@ -11,7 +11,7 @@
     <IsPackable>true</IsPackable>
     <PackageId>Radzen.Blazor</PackageId>
     <Product>Radzen.Blazor</Product>
-    <Version>8.7.5</Version>
+    <Version>8.6.3</Version>
     <Copyright>Radzen Ltd.</Copyright>
     <Authors>Radzen Ltd.</Authors>
     <Description>Radzen Blazor is the most sophisticated free UI component library for Blazor, featuring 100+ native components including DataGrid, Scheduler, Charts, and advanced theming with full support for Material Design and Fluent UI.</Description>

Radzen.Blazor/RadzenAccordion.razor

@@ -19,11 +19,9 @@
         if (!item.Visible)
             continue;
 
-        var accordionId = $"{GetId()}-accordiontab-{items.IndexOf(item)}";
         <div @ref="@item.Element" id="@item.GetItemId()" @attributes="item.Attributes" class="@item.GetItemCssClass()" style="@item.Style" @onkeydown:stopPropagation>
-            <button type="button" @onclick="@((args) => SelectItem(item))" aria-label="@ItemAriaLabel(i, item)" title="@ItemTitle(i, item)" role="tab"
-                    id="@accordionId" aria-controls="@($"{accordionId}-content")" aria-expanded="@(item.GetSelected() ? "true" : "false")"
-                    aria-selected="@(item.GetSelected() ? "true" : "false")" tabindex="@(item.GetSelected() ? "0" : "-1")">
+            <a @onclick="@((args) => SelectItem(item))" aria-label="@ItemAriaLabel(i, item)" title="@ItemTitle(i, item)" @onclick:preventDefault="true" role="tab" 
+               id="@($"rz-accordiontab-{items.IndexOf(item)}")" aria-controls="@($"rz-accordiontab-{items.IndexOf(item)}-content")" aria-expanded="true">
                 <span class="@ToggleIconClass(item)">keyboard_arrow_down</span>
                 @if (!string.IsNullOrEmpty(item.Icon))
                 {
@@ -37,10 +35,10 @@
                 {
                     <span>@item.Text</span>
                 }
-            </button>
+            </a>
         </div>
         <Expander Expanded=@item.GetSelected() role="tabpanel"
-                id="@($"{accordionId}-content")" aria-labelledby="@accordionId">
+                id="@($"rz-accordiontab-{items.IndexOf(item)}-content")" aria-labelledby="@($"rz-accordiontab-{items.IndexOf(item)}")">
             <div class="rz-accordion-content" @onkeydown:stopPropagation>
                 @item.ChildContent
             </div>