The ThreadingModel property specifies the threading model used by in-process COM classes, including: Apartment - components ...

The ThreadingModel property specifies the threading model used by in-process COM classes, including: Apartment - components may be entered by one and only one thread. Common data held by these kinds of object servers must be protected against thread collisions since the object server supports multiple components. Each component can be entered simultaneously by different threads. Free - components place no restrictions on which threads or how many threads can enter the object. The object cannot contain thread-specific data and must protect its data from simultaneous access by multiple threads. Free-threaded components however, cannot be accessed by apartment threads directly, and calls to them are marshaled across from the client apartment. Both - components that can be used in either apartment-threaded or free-threaded modes. These components can be entered by multiple threads, protect their data from thread collisions, and do not contain thread-specific data. If this property is NULL, then no threading model is used. The component is created on the main thread of the client and calls from other threads are marshaled to this thread. Please refer to the Platform Software Development Kit for information on the various threading models.