About COM threading architecture & the Apartment.

Tuesday, July 28, 2009

Definition of Apartment

• COM objects in the process are divided into groups called apartments.

• A COM object lives in exactly one apartment, in the sense that its methods com legally be directly called only by a thread that belongs to that apartment. Any other thread that wants to call the object must go through a proxy.

COM Threading Architecture is as below.

[PROCESS]

[Object]

Apartment

Difference between Single Threaded and Multi Threaded apartment

Single Threaded

• Single threaded apartment consist of exactly one thread.

• All COM objects that live in a single threaded apartment. COM receives method calls only from the one thread that belongs to that apartment.

• All method calls to a com objects are synchronized with the windows message queue.

• A process with a single thread is simply a special case of this mode.

Multi Threaded

• Multi threaded apartment consist of one or more thread.

• All COM objects can receive methods calls directly from any of the threads that belong to the multi-threaded apartments.

• All method calls to COM objects are synchronized by the objects themselves.

• Threads in MTA use model called free threading.