Invalid cast from 'WinRT.IInspectable' to 'System.Collections.Generic.IEnumerable`1[Windows.Storage.IStorageFile]' using WinUI

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 WinUI on Desktop, not with UWP.

Steps to reproduce the bug

1. Download the WinUI 3 Project Templates extension
2. Create a new C# “Blank App, Packaged (WinUI in Desktop)” via the template (two projects will be created, one for the code, one for the package)
3. Possibly: edit the non-package project file to make sure it lists the correct TargetFramework for me it lists net5.0-windows10.0.18362.0 while I only have 10.0.19041.0 installed. (Issue: #4062)
4. 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();
    }
});

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

Expected behavior Should work without exception.

Version Info [Microsoft.WinUI 3.0.0-preview3.201113.0]

Windows app type:

UWP	Win32
No	Yes

Windows 10 version	Saw the problem?
Insider Build (20H2: 19042.746)	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

[stevenwdv]

@StephenLPeters No, I actually explicitly mentioned that issue in the steps but it’s not the issue I report here. Please read my report again more slowly 😉

[github-actions[bot]]

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