Fix NullReferenceException when extracting 7z empty-stream entries

[Copilot]

Extracting 7z archives containing zero-byte files throws NullReferenceException in SevenZipArchive.GetEntryStream(). Empty-stream entries (files with HasStream == false) have Folder = null, but the code dereferenced folder! without checking.

Changes

• Core fix: Added null check in SevenZipArchive.GetEntryStream() to return Stream.Null for empty-stream entries, consistent with directory handling
• Test coverage: Added 7Zip.EmptyStream.7z test archive (4 empty files) and verification test

var folder = entry.FilePart.Folder;

// If folder is null (empty stream entry), return empty stream
if (folder is null)
{
    return CreateEntryStream(Stream.Null);
}

// ... existing folder stream handling

Empty-stream entries are a valid 7z construct where files contain no compressed data. The Folder property is null for such entries by design (see SevenZipFilePart constructor).

Original prompt

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This section details on the original issue you should resolve

<issue_title>NullReferenceException when extracting 7z with empty-stream entry via WriteToDirectory</issue_title>
<issue_description>## Summary
Extracting a specific .7z archive throws NullReferenceException in SharpCompress 0.46.0 when calling
WriteToDirectory.

Repro project

https://github.com/YoshihiroIto/SharpCompressRepro_SevenZip_Extraction_Failure

Steps to reproduce

1. Clone the repro project.
2. Run:

   dotnet run
   

3. The app uses archive00.7z and calls archive.WriteToDirectory(...).

Actual behavior

A System.NullReferenceException is thrown from:

• SharpCompress.Common.SevenZip.ArchiveDatabase.GetFolderStream(...)
• called via SharpCompress.Archives.SevenZip.SevenZipArchive.SevenZipReader.GetEntryStream()

Expected behavior

Extraction should succeed without exception.
For an empty-stream file entry, SharpCompress should create a zero-byte file (or otherwise handle it safely), not
crash.

Environment

• SharpCompress: 0.46.0
• .NET SDK: 10.0.103
• OS: Windows (repro project environment)

Notes

This appears related to 7z entries where HasStream == false (valid empty file entry).
In that case, folder-based stream handling may dereference a null folder.
</issue_description>

<agent_instructions>Redo #1218 but on release</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes NullReferenceException when extracting 7z with empty-stream entry via WriteToDirectory #1217

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[kiloconnect]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• src/SharpCompress/Archives/SevenZip/SevenZipArchive.cs
• src/SharpCompress/IO/SharpCompressStream.cs
• tests/SharpCompress.Test/SevenZip/SevenZipArchiveTests.cs

[YoshihiroIto]

Hi @adamhathcock, thanks for getting this fix merged quickly (especially on the release branch).

I noticed PR #1219 is effectively a rework/port of my original PR #1218 (“redo #1218 but on release”), and #1218 ended up closed without attribution carrying over.

Also, as a factual detail, the reproduction test data file 7Zip.EmptyStream.7z used here is the exact file I provided in #1218.

Would you mind adding a short credit/reference for traceability? For example, adding a line to the PR description like:
“Originally contributed by @YoshihiroIto in #1218 (ported to release in #1219).”

Thanks!

[adamhathcock]

I wasn't trying to not credit you, I just let the robots do their thing referencing your PR but on the release branch and I manually merged the fix into master

Originally contributed by @YoshihiroIto in #1218 (ported to release in #1219).

[YoshihiroIto]

Hi @adamhathcock,

Thanks for clarifying — much appreciated!
And thanks for adding the attribution line. 🙏