Multiple Threads in Applications

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If you programmed in versions of VB prior to .NET, you might know that VB supported multiple threads within a COM container, such as MTS or COM+. Well, although multiple threads were supported by VB5/6, the threading model they supported was Single Threaded Apartments (STA). If you come from Visual C++ then you'd have options to build both MTA (Multi Threaded Apartments) and STA applications. However, the .NET Framework does not retain the concept of Apartments and it manages all of the threads within AppDomains. By default, all .NET applications are multithreaded and any code can access any object at any time. Thus, we have to be very careful with static resources in the managed code.

The .NET Framework supports both managed and unmanaged threads and all the Win32 threading models such as STA and MTA. When you are trying to access COM components from managed code, unmanaged threads are created by the legacy COM components. Threads in the .NET Framework are created using the Thread object, whether managed or unmanaged.

If you have ever programmed multithreaded programs using the Win32 APIs, you may remember that Win32 supported user-interface threads and worker threads. As you learned in Chapter 1, the threading names have now changed into Apartment Model Threading and Free Threading respectively. The .NET Framework supports two basic threading models, which are Apartment Model Threaded or Single Threaded Apartment (STA) components, and Free Threaded or Multi Threaded Apartment (MTA) components. When we create a thread in .NET, by default it is an MTA thread.

You should only use the STA threading model when you're going to access STA-based COM components such as VB6 COM components. Otherwise, you shouldn't mark the current thread as STA, since it involves a significant performance hit to the application.

To reiterate what you learned earlier, an apartment is the logical container within the AppDomain for sharing threads in the same context. Objects reside inside an AppDomain and the context is created when an object is created during the activation process.

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