Apartment Models

Occasionally, an InProc Server may have multiple apartments. There are 3 types of of apartment:

• Single-Threaded Apartment (STA)
• Multithreaded Apartment (MTA)
• Thread-Neutral Apartment (TNA)

One process may have multiple STA apartments, a single MTA and a single TNA, in any combination. If you don’t specify anything, by default both server and client go into the ‘main’ STA. This was the only option available in Win16.

A COM server can live in an STA, MTA or TNA apartment and a COM client can live in an STA or MTA apartment. It’s a complicated subject, which starts off on the wrong foot with inconsistent terminology.

The apartment model of a COM server is specified in the class’s registry entry under HKEY_CLASSES_ROOT\CLSID\<GUID>\InprocServer32\ThreadingModel. The options are

The apartment model of a COM client is specified when the runtime is initialized.

Ideally they server and client live in the same apartment, in which case the method calls are direct. Communication with a COM server in another apartment incurs some overhead.

To describe these models very briefly -

• An STA is implemented as a window named OleMainThreadWndClass and its thread is running a message loop.
  • There is one instance of this window for each STA.
  • If both server and client are in the same STA, there is one window.
• The MTA has a thread pool.
• The TNA has no threads; instead, it uses the caller’s thread.

Calling from one apartment into another may incur overhead. Depending on the scenario, the client may call the server in another apartment using direct pointer access, using the proxy from the class’s typelib, or generate a hidden “lightweight” proxy.

As a rule of thumb - keep everything in the same apartment, unless there is a compelling reason to do otherwise.

• If the COM server touches UI, like and ActiveX control, leave ThreadingModel undefined or set it to Apartment, to keep the server on the window’s UI thread.
• Otherwise use Both. In all cases, this will allow direct pointer access.

In the unusual case where there are multiple apartments, you can run into a problem of fairness. If a client in apartment STA1 instantiates an InProc server, and then a client in STA2 instantiates the same server, the server object will end up in STA1. The first client will have direct access to it and the second will have to communicate over a proxy/stub. (Talk about a first-world problem…) This is where the TNA comes into the picture. It’s a neutral place where both clients will have indirect access to the server, but it will be running on their threads.

Strangely, the COM+ literature recommends Neutral as the default. However, the registry on my dev box shows that TNA servers are exceedingly rare in practice, appearing only in very large applications like SQL Server and Visual Studio.

Even though different apartments live in the same process, i.e. address space, interface pointers can’t be shared between apartments. To send a pointer over, place it on a stream with CoMarshalInterThreadInterfaceInStream and retrieve it with CoGetInterfaceAndReleaseStream. To coordinate these calls, generally you’ll need to use a kernel event.

An OutProc Server may also have multiple apartments.

.NET COM clients can be either MTA or STA.

References

• COM+ Threading Models - Win32 apps | Microsoft Learn
• CoMarshalInterThreadInterfaceInStream function (combaseapi.h) - Win32 apps | Microsoft Learn
• CoGetInterfaceAndReleaseStream function (combaseapi.h) - Win32 apps | Microsoft Learn