Fix 7z encryption detection for streamless files

docs/API.md

@@ -831,3 +831,7 @@ using (var reader = ReaderFactory.OpenReader(stream))
 - [USAGE.md](USAGE.md) - Complete code examples
 - [FORMATS.md](FORMATS.md) - Supported formats
 - [PERFORMANCE.md](PERFORMANCE.md) - API selection guide
+## 7z Encryption Detection
+
+For 7z archives, `IsEncrypted` is true when any entry has an AES-encrypted data stream.
+Streamless empty files and directories are not encrypted. Archives without files return false.

src/SharpCompress/Archives/SevenZip/SevenZipArchive.cs

@@ -98,7 +98,7 @@ public partial class SevenZipArchive : AbstractArchive<SevenZipArchiveEntry, Sev
             .GroupBy(x => x.FilePart.Folder)
             .Any(folder => folder.Skip(1).Any());
 
-    public override bool IsEncrypted => Entries.First(x => !x.IsDirectory).IsEncrypted;
+    public override bool IsEncrypted => Entries.Any(x => x.IsEncrypted);
 
     public override long TotalSize =>
         _database?._packSizes.Aggregate(0L, (total, packSize) => total + packSize) ?? 0;

src/SharpCompress/Common/SevenZip/SevenZipFilePart.cs

@@ -119,7 +119,5 @@ internal class SevenZipFilePart : FilePart
         };
     }
 
-    internal bool IsEncrypted =>
-        !Header.IsDir
-        && Folder?._coders.FindIndex(c => c._methodId._id == CMethodId.K_AES_ID) != -1;
+    internal bool IsEncrypted => !Header.IsDir && Folder?.IsEncrypted() == true;
 }

tests/SharpCompress.Test/SevenZip/SevenZipEncryptionDetectionTests.cs

@@ -0,0 +1,82 @@
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using SharpCompress.Archives.SevenZip;
+using SharpCompress.Common.SevenZip;
+using SharpCompress.Crypto;
+using SharpCompress.Writers.SevenZip;
+using Xunit;
+
+namespace SharpCompress.Test.SevenZip;
+
+public class SevenZipEncryptionDetectionTests
+{
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task SevenZipArchive_EmptyFirstFile_EncryptionDetection(bool encrypted)
+    {
+        using var output = new MemoryStream();
+        using (var writer = new SevenZipWriter(output, new SevenZipWriterOptions()))
+        {
+            using var empty = new MemoryStream();
+            using var data = new MemoryStream(new byte[] { 1, 2, 3 });
+            writer.Write("empty.txt", empty, null);
+            writer.Write("data.txt", data, null);
+        }
+        output.Position = 0;
+        using var archive = (SevenZipArchive)SevenZipArchive.OpenArchive(output);
+        var entries = archive.Entries.Cast<SevenZipArchiveEntry>().ToList();
+        if (encrypted)
+        {
+            // Only metadata is under test; do not decode after adding the AES marker.
+            entries[1]
+                .FilePart.Folder!._coders.Add(new CCoderInfo { _methodId = CMethodId.K_AES });
+        }
+        Assert.False(entries[0].IsEncrypted);
+        Assert.Same(Stream.Null, entries[0].OpenEntryStream());
+        Assert.Equal(encrypted, entries[1].IsEncrypted);
+        Assert.Equal(encrypted, archive.IsEncrypted);
+        Assert.Equal(encrypted, await archive.IsEncryptedAsync());
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void SevenZipArchive_NoFiles_IsNotEncrypted(bool directory)
+    {
+        using var stream = new MemoryStream();
+        // Build reader fixtures directly so this PR does not depend on the writer fix in #1440.
+        using var header = new MemoryStream();
+        ArchiveHeaderWriter.WriteRawHeader(
+            header,
+            null,
+            new SevenZipFilesInfoWriter
+            {
+                Entries = directory
+                    ? new[]
+                    {
+                        new SevenZipWriteEntry
+                        {
+                            Name = "dir",
+                            IsDirectory = true,
+                            IsEmpty = true,
+                            Attributes = 0x10,
+                        },
+                    }
+                    : System.Array.Empty<SevenZipWriteEntry>(),
+            }
+        );
+        var bytes = header.ToArray();
+        var crc = Crc32Stream.Compute(
+            Crc32Stream.DEFAULT_POLYNOMIAL,
+            Crc32Stream.DEFAULT_SEED,
+            bytes
+        );
+        SevenZipSignatureHeaderWriter.WriteFinal(stream, 0, (ulong)bytes.Length, crc);
+        stream.Write(bytes, 0, bytes.Length);
+        stream.Position = 0;
+        using var archive = SevenZipArchive.OpenArchive(stream);
+        Assert.False(archive.IsEncrypted);
+    }
+}