Fix NullReferenceException when extracting 7z empty-stream entries

src/SharpCompress/Archives/SevenZip/SevenZipArchive.cs

@@ -164,6 +164,12 @@ protected override EntryStream GetEntryStream()
 
             var folder = entry.FilePart.Folder;
 
+            // If folder is null (empty stream entry), return empty stream
+            if (folder is null)
+            {
+                return CreateEntryStream(Stream.Null);
+            }
+
             // Check if we're starting a new folder - dispose old folder stream if needed
             if (folder != _currentFolder)
             {

src/SharpCompress/IO/SharpCompressStream.cs

@@ -206,8 +206,22 @@ public override void Flush()
         throw new NotSupportedException();
     }
 
-    public override long Length =>
-        _isPassthrough ? stream.Length : throw new NotSupportedException();
+    public override long Length
+    {
+        get
+        {
+            if (_isPassthrough)
+            {
+                return stream.Length;
+            }
+
+            if (_ringBuffer is not null)
+            {
+                return _ringBuffer.Length;
+            }
+            throw new NotSupportedException();
+        }
+    }
 
     public override long Position
     {

src/SharpCompress/packages.lock.json

@@ -216,9 +216,9 @@
     "net10.0": {
       "Microsoft.NET.ILLink.Tasks": {
         "type": "Direct",
-        "requested": "[10.0.0, )",
-        "resolved": "10.0.0",
-        "contentHash": "kICGrGYEzCNI3wPzfEXcwNHgTvlvVn9yJDhSdRK+oZQy4jvYH529u7O0xf5ocQKzOMjfS07+3z9PKRIjrFMJDA=="
+        "requested": "[10.0.2, )",
+        "resolved": "10.0.2",
+        "contentHash": "sXdDtMf2qcnbygw9OdE535c2lxSxrZP8gO4UhDJ0xiJbl1wIqXS1OTcTDFTIJPOFd6Mhcm8gPEthqWGUxBsTqw=="
       },
       "Microsoft.NETFramework.ReferenceAssemblies": {
         "type": "Direct",
@@ -264,9 +264,9 @@
     "net8.0": {
       "Microsoft.NET.ILLink.Tasks": {
         "type": "Direct",
-        "requested": "[8.0.22, )",
-        "resolved": "8.0.22",
-        "contentHash": "MhcMithKEiyyNkD2ZfbDZPmcOdi0GheGfg8saEIIEfD/fol3iHmcV8TsZkD4ZYz5gdUuoX4YtlVySUU7Sxl9SQ=="
+        "requested": "[8.0.23, )",
+        "resolved": "8.0.23",
+        "contentHash": "GqHiB1HbbODWPbY/lc5xLQH8siEEhNA0ptpJCC6X6adtAYNEzu5ZlqV3YHA3Gh7fuEwgA8XqVwMtH2KNtuQM1Q=="
       },
       "Microsoft.NETFramework.ReferenceAssemblies": {
         "type": "Direct",

tests/SharpCompress.Test/SevenZip/SevenZipArchiveTests.cs

@@ -4,6 +4,7 @@
 using SharpCompress.Archives;
 using SharpCompress.Archives.SevenZip;
 using SharpCompress.Common;
+using SharpCompress.Common.SevenZip;
 using SharpCompress.Factories;
 using SharpCompress.Readers;
 using Xunit;
@@ -344,4 +345,53 @@ public void SevenZipArchive_Solid_VerifyStreamReuse()
         // The critical check: within a single folder, the stream should NEVER be recreated
         Assert.Equal(0, streamRecreationsWithinFolder); // Folder stream should remain the same for all entries in the same folder
     }
+
+    [Fact]
+    public void SevenZipArchive_EmptyStream_WriteToDirectory()
+    {
+        // This test specifically verifies that archives with empty-stream entries
+        // (files with size 0 and no compressed data) can be extracted without throwing
+        // NullReferenceException. This was previously failing because the folder was null
+        // for empty-stream entries.
+        var testArchive = Path.Combine(TEST_ARCHIVES_PATH, "7Zip.EmptyStream.7z");
+        using var archive = SevenZipArchive.OpenArchive(testArchive);
+
+        var emptyStreamFileCount = 0;
+        foreach (var entry in archive.Entries)
+        {
+            if (!entry.IsDirectory)
+            {
+                // Verify this is actually an empty-stream entry (HasStream == false)
+                var sevenZipEntry = entry as SevenZipEntry;
+                if (sevenZipEntry?.FilePart.Header.HasStream == false)
+                {
+                    emptyStreamFileCount++;
+                }
+
+                // This should not throw NullReferenceException
+                entry.WriteToDirectory(SCRATCH_FILES_PATH);
+            }
+        }
+
+        // Ensure we actually tested empty-stream entries
+        Assert.True(
+            emptyStreamFileCount > 0,
+            "Test archive should contain at least one empty-stream entry"
+        );
+
+        // Verify that empty files were created
+        var extractedFiles = Directory.GetFiles(
+            SCRATCH_FILES_PATH,
+            "*",
+            SearchOption.AllDirectories
+        );
+        Assert.NotEmpty(extractedFiles);
+
+        // All extracted files should be empty (0 bytes)
+        foreach (var file in extractedFiles)
+        {
+            var fileInfo = new FileInfo(file);
+            Assert.Equal(0, fileInfo.Length);
+        }
+    }
 }