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COM Apartments

There are three COM Apartment models:
Single-threaded apartments STA COM serializes calls between apartments through a Windows message loop. CoInitializeEx(NULL, COINIT_APARTMENTTHREADED)
Multi-threaded apartments MTA Serialization is not provided by COM, instead COM-components are expected to provide their own synchronization. CoInitializeEx(NULL, COINIT_MULTITHREADED)
Neutral-threaded apartments NTA Windows 2000 only
A process may have multiple STAs but only one MTA.
Every COM Object belongs to exactly one apartment.

See also

The System.Thrading.ApartmentState enum.
The value ThreadingModel under the registry key HKEY_CLASSES_ROOT\CLSID\{GUID}\InprocServer32
The PowerShell command line parameters -STA and -MTA.

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