C# Definition:

[StructLayout(LayoutKind.Sequential)]
struct MULTI_QI
{
    [MarshalAs(UnmanagedType.LPStruct)]  public Guid pIID;
    [MarshalAs(UnmanagedType.Interface)] public object pItf;
    public int hr;
}

User-Defined Field Types:

None.

Notes:

Marshal.SizeOf() refuses to deal the pIID member as it is specified, perhaps because it cannot manage the pointer.