19. What is a multi-threaded apartment (MTA)? Single-threaded apartment (STA)?

This is pretty difficult question to describe shortly. Anyway, apartments were introduced by Microsoft in NT 3.51 and late Windows 95 to isolate the problem of running legacy non-thread safe code into multithreaded environment. Each thread was “encapsulated” into so called single-threaded apartment. The reason to create an object in apartment is thread-safety. COM is responsible synchronize access to the object even if the object inside of the apartment is not thread-safe. Multithreaded apartments (MTA, or free threading apartment) were introduced in NT 4.0. Idea behind MTA is that COM is not responsible to synchronize object calls between threads. In MTA the developer is responsible for that. See “Professional DCOM Programming” of Dr. Grimes et al. or “Essential COM” of Don Box for the further discussion on this topic.