[COM] Is the runtime supposed to call ComWrappers.ComputeVTables if the object is an RCW?

[savatia]

Hi folks, I'm trying to pass my interface IFoo to an exported function [DllImport("CppProject")] static extern int FooFunction(IFoo foo); but I'm hitting a slight issue. During marshalling, the runtime asks my ComWrapper for the Vtable. I'm able to generate the Vtbale and pass it to the runtime. Problem is, when the exported FooFunction is called, it passes a completely made up pointer to the IFoo instance, but the Vtable is correct. What am I doing wrong?

[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("C859BA31-821B-4947-A04F-5C30C6A94B82")]
public interface IFoo
{
    [PreserveSig]
    int Bar([In][MarshalAs(UnmanagedType.LPStr)] string Message);
}

public class FooComWrapper : IFoo
{
    public struct IFooVtbl
    {
        public IntPtr Bar;
    }

    public IntPtr Instance { get; }
    public IFooVtbl Vtbl { get; }

    public delegate int BarDelegate(IntPtr ptrThis, [MarshalAs(UnmanagedType.LPStr)] string Message);
    public BarDelegate BarFunction { get; set; }

    public FooComWrapper(IntPtr ptr)
    {
        Instance = ptr;
        unsafe
        {
            Vtbl = Marshal.PtrToStructure<IFooVtbl>(new nint(*(void**)ptr.ToPointer()));
            BarFunction = Marshal.GetDelegateForFunctionPointer<BarDelegate>(Vtbl.Bar);
        }
    }

    public int Bar(string Message)
    {
        return BarFunction(Instance, Message);
    }
}

public class FooComWrappers : ComWrappers
{
    protected override unsafe ComInterfaceEntry* ComputeVtables(object obj, CreateComInterfaceFlags flags, out int count)
    {
        ComInterfaceEntry* entry = null;
        count = 0;

        if (obj is FooComWrapper foo)
        {
            // What do I do here lmao?
            count = 1;
            entry = (ComInterfaceEntry*)RuntimeHelpers.AllocateTypeAssociatedMemory(typeof(FooComWrappers), sizeof(ComInterfaceEntry));
            entry->Vtable = *(IntPtr*)foo.Instance.ToPointer();
            entry->IID = typeof(IFoo).GUID;
        }

        return entry;
    }

    protected override object? CreateObject(nint externalComObject, CreateObjectFlags flags)
    {
        return new FooComWrapper(externalComObject);
    }

    protected override void ReleaseObjects(IEnumerable objects)
    {
        throw new NotImplementedException();
    }
}

public static class Program
{
    [DllImport("CppProject")] static extern int FooFactory(out IntPtr ptr);
    [DllImport("CppProject")] static extern int FooFunction(IFoo foo);

    [SupportedOSPlatform("Windows")]
    public static void Main(string[] args)
    {
        ComWrappers.RegisterForMarshalling(new FooComWrappers());
        if (FooFactory(out IntPtr ptrFoo) == 0)
        {
            if (Marshal.GetObjectForIUnknown(ptrFoo) is IFoo foo)
            {
                foo.Bar("RCW");
                _ = FooFunction(foo);      // <<<<<< Pointer passed here to unmanaged function is incorrect but computed vtable is correct
            }
        }
    }
}