c# - Invalid cast from 'WinRT.IInspectable' to 'System.Collections.Generic.IEnumerable`1[Windows.Storage.IStorageFile]'

Question

Why does the following Desktop UWP (or WinUI or WinRT? I get confused by the terminology) code compile but throw an exception?

var folder = await Windows.Storage.StorageFolder.GetFolderFromPathAsync(@"pathofolder");
var files  = await folder.GetFilesAsync(); //IReadOnlyList<Windows.Storage.StorageFile>)

//Exception: Invalid cast from 'WinRT.IInspectable' to 'System.Collections.Generic.IEnumerable`1[Windows.Storage.IStorageFile]'.
var l1 = ((IReadOnlyList<Windows.Storage.IStorageFile>) files).ToList(); //Even crashes if we add e.g. .Select(_=>42) in-between.

Note the cast from StorageFile to IStorageFile (a more nasty bug, which is what I had, is a method taking a list of the latter). Is there something wrong with casting to interfaces in this context, or is this a bug? Normally it works fine, e.g. ((IStorageFile) files[0]).Name.

This only happens when using C#/WinRT on Desktop. Steps to reproduce:

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. I guess I'll report this as a bug in a moment. [Seems to be already reported: GitHub issue]
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();
    }
});

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

question from:https://stackoverflow.com/questions/65928154/invalid-cast-from-winrt-iinspectable-to-system-collections-generic-ienumerabl

Answer 1

Waitting for answers