WinUI 2's XamlMetadataProvider provides incorrect UnderlyingType for System.Windows.Input.ICommand

Describe the bug WinUI 2’s metadata provider, Microsoft.UI.Xaml.XamlTypeInfo.XamlControlsXamlMetaDataProvider, contains incorrect type information for System.Windows.Input.ICommand type which can cause crashes.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Create a new blank C# UWP app
2. Add a reference to the Microsoft.UI.Xaml/WinUI 2 NuGet package (and add XamlControlsResources to App.xaml as normal)
3. Add the following code to the app’s constructor in App.xaml.cs:

        public App()
        {
            this.InitializeComponent();
            this.Suspending += OnSuspending;

            // Get the IXamlType for ICommand from WinUI 2's metadata provider, which is incorrectly created
            // with an invalid UnderlyingType property causing a crash when accessed
            var winuiXMP = new Microsoft.UI.Xaml.XamlTypeInfo.XamlControlsXamlMetaDataProvider();
            var winuiType = winuiXMP.GetXamlType("System.Windows.Input.ICommand");
            var winuiUnderlyingType = winuiType.UnderlyingType; //Crashes
        }

1. Run the app
2. The app will crash on the last line when attempting to access winuiType.UnderlyingType

In reality, this can occur in the following setup:

• A C# application uses a property/field of type System.Windows.Input.ICommand in markup
• The C# application references WinUI 2
• The C# application references a C# control library, and the control library does not use System.Windows.Input.ICommand anywhere

Expected behavior winuiUnderlyingType.UnderlyingType returns the System.Type corresponding to System.Windows.Input.ICommand (or maybe winuiXMP.GetXamlType("System.Windows.Input.ICommand") should return null - see the “Additional Context” section)

Screenshots

Version Info

NuGet package version: [Microsoft.UI.Xaml 2.5.0]

Windows app type:

UWP	Win32
Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context

The WinUI 2 metadata provider might need to provide type information for Windows.UI.Xaml.Input.ICommand instead of System.Windows.Input.ICommand, since the Windows.UI.Xaml version is what exists in native code where WinUI 2’s metadata provider is written and should be projected to the System.Windows version.

[RealTommyKlein]

@michael-hawker - it’s definitely the same issue (the callstack contains Windows.UI.Xaml.Markup.IXamlType.get_UnderlyingType), and this bug would affect both Debug and Release builds.

[RealTommyKlein]

Yes, I believe the solution under the Additional Context section could resolve it (WinUI 2’s metadata provider has an IXamlType for ICommand, but its IXamlType.FullName property should be in the native Windows.UI.Xaml.Input.ICommand namespace instead of the managed System.Windows.Input.ICommand namespace).