Support Windows shell virtual files in drag and drop by YoshihiroIto · Pull Request #21907 · AvaloniaUI/Avalonia

YoshihiroIto · 2026-07-31T16:34:01Z

What does the pull request do?

Adds Windows shell virtual-file support to the OLE data-transfer backend. Applications can now receive files dragged from sources that do not expose a filesystem path yet, including entries dragged from ZIP folders in Windows Explorer.

Virtual entries are exposed through the existing DataFormat.File / IStorageFile API, so no new public API is required.

What is the current behavior?

OleDataObjectToDataTransferWrapper only recognizes physical files supplied through CF_HDROP. A Windows shell source that supplies FileGroupDescriptorW plus indexed FileContents therefore does not produce DataFormat.File items.

What is the updated/expected behavior with this PR?

FileGroupDescriptorW and FileContents are recognized as file data.
Each descriptor is exposed as an IStorageFile, including its name and optional size.
FileContents supports both IStream and HGLOBAL providers.
Streams can be consumed on a worker thread without violating COM apartment ownership.
IDataObjectAsyncCapability keeps the source operation alive after Drop returns and is completed when all returned storage items are disposed.

How was the solution implemented (if it's not obvious)?

The Win32 backend parses the shell FILEGROUPDESCRIPTORW payload and requests each indexed FileContents entry. COM IStream and IDataObjectAsyncCapability interfaces are explicitly marshaled between apartments. The unmarshaled stream is owned by a generated MicroCom proxy, retaining trimming and NativeAOT compatibility while also providing finalizer-backed COM lifetime management. IStream.Read preserves the raw HRESULT so the documented S_FALSE end-of-stream result is handled correctly, and Length/Seek/Position delegate to the underlying stream.

The implementation requests TYMED_ISTREAM first and retries with TYMED_HGLOBAL, because some valid IDataObject implementations provide only the latter.

The non-file transfer item now receives only its actual non-file formats, preventing a duplicate DataFormat.File item when virtual files are present. A failure to materialize one virtual file is logged as a warning and does not prevent the remaining files from being returned.

Validation performed:

Avalonia.Win32 Release builds for net8.0 and net10.0: succeeded.
Avalonia.IntegrationTests.Win32: 242 passed, including virtual-file contents/size, truncated descriptor input, and completing IDataObjectAsyncCapability from a worker thread.
win-x64 NativeAOT smoke publish with OleVirtualFileData rooted: succeeded, and the generated executable ran successfully. The root-all harness surfaces an existing Enum.GetValues(Type) IL3050 warning outside this implementation.

Checklist

Added unit tests (if possible)?
Added XML documentation to any related classes?
Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

None.

avaloniaui-bot · 2026-07-31T17:13:35Z

You can test this PR using the following package version. 12.2.999-cibuild0068018-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

avaloniaui-bot · 2026-08-01T08:21:46Z

You can test this PR using the following package version. 12.2.999-cibuild0068022-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

Copilot

Pull request overview

This PR adds Windows shell virtual-file support to Avalonia’s Win32 OLE data-transfer backend, enabling drag-and-drop of files that don’t yet have filesystem paths (e.g., items dragged out of ZIP folders in Windows Explorer) by exposing them through the existing DataFormat.File / IStorageFile APIs.

Changes:

Add OleVirtualFileData to parse FileGroupDescriptorW and fetch indexed FileContents as IStorageFile instances, including COM apartment-safe marshaling for IStream.
Extend OleDataObjectToDataTransferWrapper to surface virtual files as DataFormat.File and avoid duplicating file formats in the non-file transfer item.
Add Win32 integration tests and grant test assembly access via InternalsVisibleTo; add IDataObjectAsyncCapability to Win32 COM IDL.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/Avalonia.IntegrationTests.Win32/OleVirtualFileDataTests.cs	Adds integration tests for descriptor parsing, virtual file exposure, and async operation completion.
src/Windows/Avalonia.Win32/Win32Com/win32.idl	Introduces `IDataObjectAsyncCapability` COM interface definition for async drag/drop lifetime management.
src/Windows/Avalonia.Win32/OleVirtualFileData.cs	Implements virtual-file parsing, `FileContents` extraction (IStream/HGLOBAL), and async operation lifetime handling.
src/Windows/Avalonia.Win32/OleDataObjectToDataTransferWrapper.cs	Detects virtual-file formats and materializes them as `DataFormat.File` items without duplicating formats in non-file items.
src/Windows/Avalonia.Win32/Avalonia.Win32.csproj	Adds `InternalsVisibleTo` for the Win32 integration test assembly.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

avaloniaui-bot · 2026-08-03T09:42:35Z

You can test this PR using the following package version. 12.2.999-cibuild0068036-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

avaloniaui-bot · 2026-08-04T01:35:09Z

You can test this PR using the following package version. 12.2.999-cibuild0068079-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

…-file-drop

YoshihiroIto · 2026-08-28T22:02:57Z

@MrJul Thank you for the review. I addressed the requested changes in 15ff497:

replaced descriptor offsets with a native-layout FILEDESCRIPTORW
reused the existing HGLOBAL helper and moved COM P/Invokes to UnmanagedMethods
switched IStream access and lifetime management to a generated MicroCom proxy
consolidated format detection into one pass and handled the smaller nits
removed the offset-based descriptor test

I also merged the latest main. Avalonia.IntegrationTests.Win32 passes 242/242, the net8/net10 Release builds succeed, and a rooted win-x64 NativeAOT smoke publish plus executable run succeed. Could you please take another look?

avaloniaui-bot · 2026-08-28T22:44:37Z

You can test this PR using the following package version. 12.2.999-cibuild0069123-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

YoshihiroIto · 2026-09-03T22:11:07Z

Follow-up for the latest review (commit dc7c913):

IStream.Read/Write/Seek now preserve the raw int HRESULT; Read accepts both S_OK and the documented S_FALSE EOF result.
ComReadStream delegates Seek and Position to the underlying IStream.
Removed MarshaledInterface; marshaled streams are held by generated MicroCom IStream proxies with an additional reference passed to CoGetInterfaceAndReleaseStream and deterministic disposal afterward.
TryCreateFile now isolates per-file failures, logs warnings, and allows remaining virtual files to be returned.

Validation after these changes: net8.0/net10.0 Release builds succeeded, Avalonia.IntegrationTests.Win32 passed 242/242, and the rooted win-x64 NativeAOT smoke publish and executable run succeeded. Please re-review when the CI checks finish.

avaloniaui-bot · 2026-09-03T22:55:56Z

You can test this PR using the following package version. 12.2.999-cibuild0069381-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

MrJul

Last round of review with a few remaining nits, this PR is looking great!

YoshihiroIto · 2026-09-04T20:05:47Z

Addressed the five remaining review comments in 72ba8a7:

Switched the virtual-file creation warning to structured logging.
Added the requested spacing around the stream members.
Renamed Complete to CompleteFile to clarify its per-file responsibility.
Implemented ComReadStream.Length through IStream.Stat(STATFLAG_NONAME) while preserving the raw HRESULT.

Validation on Windows:

Avalonia.Win32 Release build: net8.0 and net10.0 passed.
Avalonia.IntegrationTests.Win32: 242/242 passed on net10.0.
NativeAOT smoke publish and executable run: passed.

All review threads are resolved. Please re-review when the CI checks complete.

avaloniaui-bot · 2026-09-04T20:52:01Z

You can test this PR using the following package version. 12.2.999-cibuild0069443-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

MrJul

Tested against Explorer's zip handler and Outlook's attachments, with both drag-and-drop and clipboard. It works as expected.

LGTM!

avaloniaui-bot · 2026-09-07T12:43:05Z

You can test this PR using the following package version. 12.2.999-cibuild0069529-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

Add Win32 virtual file drop support

8afbce3

YoshihiroIto marked this pull request as ready for review August 1, 2026 07:35

MrJul added enhancement os-windows backport-candidate-12.1.x Consider this PR for backporting to 12.1 branch labels Aug 3, 2026

kekekeks reviewed Aug 3, 2026

View reviewed changes

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs

kekekeks requested a review from Copilot August 3, 2026 08:24

Copilot started reviewing on behalf of kekekeks August 3, 2026 08:24 View session

Copilot AI reviewed Aug 3, 2026

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Comment thread tests/Avalonia.IntegrationTests.Win32/OleVirtualFileDataTests.cs Outdated

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs

YoshihiroIto and others added 3 commits August 3, 2026 17:40

Potential fix for pull request finding

818f773

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

docs(win32): clarify virtual file disposal

5c59d9e

fix(win32): handle empty virtual file reads

b16d426

Merge branch 'main' into agent/windows-virtual-file-drop

a07f248

grokys assigned kekekeks and MrJul Aug 26, 2026

MrJul requested changes Aug 28, 2026

View reviewed changes

YoshihiroIto added 2 commits August 29, 2026 06:59

Address virtual file data review feedback

15ff497

Merge remote-tracking branch 'origin/main' into agent/windows-virtual…

b619bf4

…-file-drop

MrJul requested changes Sep 3, 2026

View reviewed changes

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs Outdated

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs Outdated

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs Outdated

Comment thread src/Windows/Avalonia.Win32/OleVirtualFileData.cs Outdated

Address virtual stream review feedback

dc7c913

MrJul requested changes Sep 4, 2026

View reviewed changes

Address final virtual stream review nits

72ba8a7

Merge branch 'main' into agent/windows-virtual-file-drop

6bcb0a4

MrJul approved these changes Sep 7, 2026

View reviewed changes

MrJul enabled auto-merge September 7, 2026 11:52

MrJul added this pull request to the merge queue Sep 7, 2026

Merged via the queue into AvaloniaUI:main with commit 4ec9cbb Sep 7, 2026
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What does the pull request do?

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

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Obsoletions / Deprecations

Fixed issues

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YoshihiroIto commented Jul 31, 2026 •

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