Bug: CsWinRT 3 undetermined-interface case relies on undefined COM behavior

[dongle-the-gadget]

Description

When CsWinRT could not determine which interface to use in the marshaller, it falls back to using IInspectable (see code below). However, performing a QueryInterface to IInspectable is undefined behavior.

CsWinRT/src/WinRT.Runtime2/InteropServices/WindowsRuntimeComWrappers.cs

Lines 439 to 445 in 228e89e

	// As a last resort, if we couldn't find any partially derived type to marshal and we also had no static type
	// information at all, we just return an opaque object. It can still be used via interfaces (including generics)
	// by doing 'IDynamicInterfaceCastable' casts on it. It will still work the same, just with lower performance.
	WindowsRuntimeObjectReference objectReference = WindowsRuntimeComWrappersMarshal.CreateObjectReference(
	externalComObject: interfacePointer,
	iid: in WellKnownWindowsInterfaceIIDs.IID_IInspectable,
	wrapperFlags: out wrapperFlags);

Steps To Reproduce

[This sample assumes that you have OneDrive installed with an account signed in and the machine runs Windows 11 version 24H2 or higher]

1. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\SyncRootManager
2. Find subkeys with the prefix OneDrive!, and copy the subkey name for any one of them.
3. Create a console project, using CsWinRT 3 and enabling unsafe code
4. Paste the following code in, replacing the sri variable with the subkey name copied above.

using Microsoft.Win32;
using System.Runtime.InteropServices;
using Windows.Storage.Provider;
using WindowsRuntime.InteropServices;

string sri = "[subkey name]";

using var key = Registry.LocalMachine.OpenSubKey($"SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Explorer\\SyncRootManager\\{sri}")!;
Guid factoryClsid = Guid.Parse((string)key.GetValue("StorageProviderStatusUISourceFactory")!);

IStorageProviderStatusUISourceFactory factory;

unsafe
{
    Marshal.ThrowExceptionForHR(PInvoke.CoCreateInstance(factoryClsid, null, 0x4, typeof(IStorageProviderStatusUISourceFactory).GUID, out void* ppv));
    factory = (IStorageProviderStatusUISourceFactory)WindowsRuntimeMarshal.ConvertToManaged(ppv)!;
    var statusUISource = factory.GetStatusUISource(sri);
    Console.WriteLine(statusUISource.GetStatusUI().QuotaUI.QuotaUsedLabel);
}

static partial class PInvoke
{
    [LibraryImport("combase")]
    public static unsafe partial int CoCreateInstance(
        in Guid rclsid,
        void* pUnkOuter,
        uint dwClsContext,
        in Guid riid,
        out void* ppv);
}

5. Run the project, observe InvalidCastException at the statusUISource line.

Expected Behavior

The console should print the storage usage.

Version Info

CsWinRT 3.0.0-preview.260319.2 (latest on NuGet)

Additional Context

This is essentially the same issue as microsoft/CsWin32#1716, but for WinRT