STAThread Attribute
If you're applying the STAThread attribute to your MC++ application's entry point, it will not work.

You're probably doing something like this:

[STAThread]
int main(void)
{
   // code here
}

The problem is that main() (or WinMain()) are not the actual executable entry points. That role is taken in MC++ applications by the CRT, via the _mainCRTStartup() (or _WinMainCRTStartup()) function.

There are two ways of getting around this:

1. If you don't use the CRT, bypass it's initialization code and use the /entry: linker option to make your own function the real entrypoint in the exe. This probably won't be a viable option for most people using MC++, though.
2. Set it directly from the code:

   Thread::CurrentThread->ApartmentState = ApartmentState::STA;