If you're calling an object living in STA, you should do it from the same thread or marshall the pointer before using it from a different thread. In the latter

If you're calling an object living in STA, you should do it from the same thread or marshall the pointer before using it from a different thread. In the latter case the STA's thread needs to run message loop to process calls to the object. If the thread is showing a modal dialog or blocked doing a lengthy operation it doesn't happen. As you can see there is nothing wrong with the object model itself, it just allows to call an object that was designed to be single-threaded from the threads other than its "home" thread.