COM Threading Models
COM Threading Models / Apartment Model
Definition: Multithreading in COM is referred to as the apartment model in COM
Apartment
The COM apartment is a conceptual entity that allows us to think about components and their clients in a logical way
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An apartment is not a thread, but a thread belongs to only one apartment.
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An apartment is not an instance of a COM object, but each COM instance belongs to only one apartment.
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A process can have one or more apartments, depending on its implementation.
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Apartments are created or entered by calling the CoInitialize or CoInitializeEx function. Each thread that calls CoInitialize(0) or CoInitializeEx( 0, COINIT_APARTMENTTHREADED )
- STA (Single Threaded Apartment) : Only One thread can join this Apartment.
- MTA (MultiThreaded Apartment): Multiple threads can join this Apartment.
Differences between STA and MTA
| No | Feature | STA | MTA |
| 1 | Synchronization provided by COM | YES | NO |
| 2 | Uses Windows message queues | YES | NO |
| 3 | Can have multiple threads in an apartment | NO | YES |
| 4 | Must marshal interface pointers between the threads in the same apartment | N/A | YES |
| 5 | Must marshal interface pointers between apartments | YES | YES |
| 6 | Must call Coinitialize() in every thread that uses a COM Service | YES | YES |
| 7 | Perfomance | Slow | Fast |
NOTE: ATL object wizard allows you to set the threading model. The values can be
| Threading Model | Description |
| Single / No Value | Object knows nothing about threads |
| Apartment | STA |
| Free | MTA |
| Both | Both STA & MTA |