Could you explain STA and MTA?

Question

Can you explain STA and MTA in your own words?

Also, what are apartment threads and do they pertain only to COM? If so, why?

Answer 1

Code that calls COM object dlls (for example, to read proprietary data files), may work fine in a user interface but hang mysteriously from a service. The reason is that as of .Net 2.0 user interfaces assume STA (thread-safe) while services assume MTA ((before that, services assumed STA). Having to create an STA thread for every COM call in a service can add significant overhead.