[mfkl]

What does the pull request do?

Fixes a NullReferenceException in ClassicDesktopStyleApplicationLifetime.StartCore when the application shuts down while the main window is being shown, e.g. when lifetime.Shutdown() or Close() is called from the main window's Opened handler.

What is the current behavior?

Window.Opened is raised synchronously inside MainWindow.Show(). If the handler shuts the application down (explicitly, or by closing the last/main window), DoShutdown cancels _cts, sets it to null and shuts down the dispatcher before StartCore reaches Dispatcher.UIThread.MainLoop(_cts.Token), which then throws a NullReferenceException.

What is the updated/expected behavior with this PR?

Start() returns normally with the exit code set by Shutdown(exitCode) (or by the Exit event handler), and Environment.ExitCode is set as before.

New tests in DesktopStyleApplicationLifetimeTests:

• Should_Return_ExitCode_When_Shutdown_From_MainWindow_Opened
• Should_Exit_When_MainWindow_Closed_From_Opened

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

StartCore now keeps the token in a local and only enters MainLoop if it hasn't been cancelled yet. A null check alone isn't enough: when shutdown happens before any dispatcher frame is pushed, InvokeShutdown completes the dispatcher shutdown immediately, and a later MainLoop call throws InvalidOperationException ("Cannot perform requested operation because the Dispatcher shut down") from PushFrame.

ShutdownRequestedEventArgs.WillExitMainLoop is still true in this case, which remains correct: Start() returns right away, so deferring to the managed side (as AvaloniaNativeApplicationPlatform does) is still the right behavior.

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

Fixes #7265

[github-actions]

NuGet packages

The packages for this pull request have been published (workflow run):

• Version: 12.2.999-cibuild0100454-alpha
• Feed URL: https://nuget-feed-all.avaloniaui.net/v3/index.json