System.AccessViolationException: 1.4.x broke creation of 2 Microsoft.Ui.Xaml.Window within Application's OnLaunched

[TomPMoleman]

Describe the bug

Up till the last 1.3 version it was ok, to create 2 instances of a Microsoft.Ui.Xaml.Window within your Application's OnLaunched method (or later down the call stack).
However, SDK 1.4.0 broke this, and you'll inevitably encounter a System.AccessViolationException!

Steps to reproduce the bug

1. Create new WinUi3 project
2. Raise 'Microsoft.WindowsAppSDK' NuGet version to at least 1.4.230822000
3. Open App.xaml.cs class
4. Adapt OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args) as to instantiate a new instance of Microsoft.Ui.Xaml.Window twice
5. Run application
6. => Please note, that shortly afterwards a System.AccessViolationException will be thrown:
   Exception thrown at 0x00007FFD2D05DE84 (Microsoft.UI.Windowing.Core.dll) in [...].exe: 0xC0000005: Access violation reading location 0x0000000000000030.

Expected behavior

Just like prior to version 1.4.0, no System.AccessViolationException shall be thrown.

Screenshots

NuGet package version

Windows App SDK 1.4.4: 1.4.231219000

Packaging type

Packaged (MSIX)

Windows version

Windows 11 version 21H2 (22000)

IDE

Visual Studio 2022

Additional context

All 1.4. SDK versions are affected.

[DarranRowe]

Do you have this problem if you run the application outside of Visual Studio?

Try disabling the in app toolbar.

I remember that an issue was introduced with 1.4 where the in app toolbar would cause a crash if multiple Xaml hosts were created on the same thread. I also don't remember seeing that this has been fixed either.

[TomPMoleman]

@DarranRowe Thanks a bunch for the hint, that indeed was the root cause of the issue!
I've spent a lot of time trying to figure out what I've been doing wrong in the code ... and the solution was that simple after all. ;)
(Still It would be nice if the issue will be resolved sometime in the future, to avoid anyone else investing the same time-consuming debugging effort in vain.)

[github-actions]

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Please view the issues below to see if they solve your problem, and if the issue describes your problem please consider closing this one. Thank you!

Closed similar issues:

• Windows.UI.Xaml Exception (#2701), similarity score: 0.75
• [WindowsAppSDK1.4 ]Exception on Close() ( AppWindow is set as parent of Xaml Window ) (#9203), similarity score: 0.71

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