Bug: Got error 'Specified cast is not valid.' on IList with AOT

[DinahK-2SO]

Description

This issue was discovered by developers developing Windows Apps with AOT enabled:
microsoft/WindowsAppSDK#4743 (comment)

The error 'Specified cast is not valid.' was thrown when developer build App with below code:

picker.FileTypeChoices.Add("Test file", [ ".txt" ]);

With new fix in CsWinRT 2.2, the error can be avoided by specifying a List type, for instance, changing above code into:

picker.FileTypeChoices.Add("Test file", new List<string> { ".txt" });

However, the original code still throws error, the difference is that the new error contains more details:

System.InvalidCastException: 'Failed to create a CCW for object of type 'System.Collections.Generic.List'1[System.String]' for interface with IID '98B9ACC1-4B56-532E-AC73-03D5291CCA90': the specified cast is not valid.'

Reporting to the CsWinRT team for further investigation.

Steps To Reproduce

1. in Visual Studio, create a blank Windows App
   
2. in the *.csproj file, add

   <WindowsSdkPackageVersion>10.0.26100.45</WindowsSdkPackageVersion>
   <PublishAot>true</PublishAot>

3. update the CsWinRT package to version 2.2.0
   
4. In MainWindows.xaml.cs, override method myButton_Click

       private void myButton_Click(object sender, RoutedEventArgs e)
       {
           var picker = new Windows.Storage.Pickers.FileSavePicker();
           WinRT.Interop.InitializeWithWindow.Initialize(picker, WinRT.Interop.WindowNative.GetWindowHandle(this));
           picker.FileTypeChoices.Add("Test file", [ ".test" ]);
           picker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.DocumentsLibrary;
           myButton.Content = "Clicked";
       }

5. Debug the app, click button, see the error
   
6. This code can run as a workaround:
   picker.FileTypeChoices.Add("Test file", new List<string> { ".txt" });
7. for your reference, the FileTypeChoices is IDictionary, whose values are IList
   

Expected Behavior

The code should run without error

Version Info

CsWinRT 2.2.0

Additional Context

No response

[Marv51]

If I understand the CsWinRT issue correctly, then the changes to make collection expressions work are not yet released and are only planned for 2.3.

---

Disclaimer: I’m not affiliated with this project or Microsoft. Just sharing thoughts based on personal experience.

[dongle-the-gadget]

It only supports a subset of collection expressions. This issue is about IList<T> which will not be supported unless if it's empty.

[Marv51]

@dongle-the-gadget I thought I saw an Analyzer in the 2.3 branch that would make this pretty clear, right?

[dongle-the-gadget]

Yes. You still can't use IList though, just that you get a warning so you don't have to face a crash at runtime.