In most earlier discussions, we assume that the server process contains only one apartment and the client process contains only one apartment. However, a process can have many apartments in COM.
Apartments are a concept added by COM to older RPC and distributed computing technologies. While it may seem like just another headache at first—another layer of complexity for you to worry about—rest assured that apartments are actually there to simplify programming and make your life easier. Briefly put, having many apartments (each running a single thread) relieves you of the need to make your COM objects thread-safe.
Since you have no control over who invokes a COM object (or when), the object normally has to be executed safely by multiple threads simultaneously. But you may not want to take the effort to make your object thread-safe, particularly if you’re adopting legacy code. Instead, you can run this object in an apartment that is occupied by a single thread, and by doing so, you have just put the thread-safe problem to bed. However, you must do a few things to support multiple apartments, and that’s what we’ll discuss in this section. We will study two apartment models that COM provides: the single-threaded apartment (STA) and multithreaded apartment (MTA) models. Each process can have many STAs, but at most one MTA (see Figure 5-16).
Figure 5-16. A process can have an ...