Object apartment vs Thread apartment

Question

I read about apartment model, but it is still confusing me when I'm trying to understand the following: whose apartment dictates the apartment model? Is it the thread's apartment or the apartment of the created object?
Consider the following scenario:

1. I've created C++ ATL COM dll that has only one interface which is ISimpleCom, and I have a class the implements that interface which has only one method test(); This class publicly inherit from CComObjectRootEx<CComSingleThreadModel> which sets STA apartment.

2. Reference this DLL in C# WinForms project, and create the SimpleCom object on a MTA thread (not the UI thread). The question is: Lets say UI thread call test() on this object, will it be marshaled to the thread created the object or it will be executed on the UI thread ?

   • If it follows the object COM apartment this call should be marshaled to the thread who created the object because it is an STA object.
   • If it follows creating thread apartment which is MTA, it should be executed on the UI thread.

Which one is the correct one?