This issue is a duplication of the microsoft/microsoft-ui-xaml#4111 that stevenwdv created and it seems a C#/WinRT projection issue.

Casting a collection of StorageFiles to a collection of IStorageFiles and using it fails at runtime (this may also be the case with other interfaces, not sure. This only happens when using Desktop WinUI 3.

Steps to reproduce the bug

1. Create a new C# "Blank App, Packaged (WinUI in Desktop)"
2. Add the following code to e.g. OnLauched in App.xaml.cs:

Task.Run(async () =>
{
    try
    {
        var folder = ApplicationData.Current.LocalFolder;
        var files  = await folder.GetFilesAsync();
        _ = ((IReadOnlyList<IStorageFile>) files).ToList();
    }
    catch (InvalidCastException ex)
    {
        Debugger.Break();
    }
});

3. Run the package project (enabling break on CLR exceptions recommended)
4. Observe the InvalidCastException in DynamicInterfaceCastableHelpers.IsInterfaceImplemented

Expected behavior
Should work without exception.

Version Info
[Microsoft.WinUI 3.0.0-preview4.210210.4]

Windows app type: