radzen-blazor/RadzenBlazorDemos/Pages/TreeDragDrop.razor

@using Microsoft.EntityFrameworkCore
@using RadzenBlazorDemos.Models.Northwind

@inherits DbContextPage

<RadzenTree Data=@employees Style="height: 300px" ItemRender="ItemRender">
    <RadzenTreeLevel TextProperty="LastName" ChildrenProperty="Employees1"
                     Expanded=@(i => true)
                     HasChildren=@(i => ((Employee)i).Employees1?.Any() == true) />
</RadzenTree>

@code {
    IEnumerable<Employee> employees;

    protected override async Task OnInitializedAsync()
    {
        await base.OnInitializedAsync();

        var allEmployees = dbContext.Employees.Include(e => e.Employees1).ToList();
        employees = allEmployees.Where(e => e.ReportsTo == null);
    }

    Employee draggedItem;

    void ItemRender(TreeItemRenderEventArgs args)
    { 
        var employee = (Employee)args.Value;

        // Allow drag of all items except the root item.
        if (employee.ReportsTo != null)
        {
            args.Attributes.Add("title", "Drag item to reorder");
            args.Attributes.Add("style", "cursor:grab");
            args.Attributes.Add("draggable", "true");
            args.Attributes.Add("ondragstart", EventCallback.Factory.Create<DragEventArgs>(this, () => 
            {
                if (draggedItem == null)
                {
                    draggedItem = employee;
                }
            }));
        }

        // Allow drop over any item including the root item.
        args.Attributes.Add("ondragover", "event.preventDefault()");
        args.Attributes.Add("ondrop", EventCallback.Factory.Create<DragEventArgs>(this, () =>
        {
            if (draggedItem != null && draggedItem != employee &&
                    draggedItem.ReportsTo != employee.EmployeeID && 
                    draggedItem.EmployeeID != employee.ReportsTo)
            {
                draggedItem.ReportsTo = employee.EmployeeID;
                dbContext.SaveChanges();
                draggedItem = null;
            }
        }));
    }
}