Threading Principles

Multiple Threads in Applications

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. 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.

STA Threading Model

An STA thread apartment works using a concept called Object-per-Client model, meaning the code that creates the STA thread apartment owns its threads.

In STA threading, all the calls to a thread will be placed in a queue and the calls will be processed one by one. Therefore, the STA thread will never execute multiple methods simultaneously. STA threads have their own private data and they don't share data between threads. This makes the threading model safe and avoids any data corruption and synchronization problems. However, this does restrict the options available to the developer, and performance suffers, as data has to be copied with every thread created.

As you can see from the diagram, AppDomain X has two STA threads, X and Y, running inside, and each of the STA Apartments has only one thread. The term Thread Affinity is used when defining the relationship between the thread and the code that creates the thread. When a call is made to an STA apartment thread, then calls between the caller and the thread are handled by the contexts in the AppDomain, and the contexts maintain the thread affinity.

MTA Threading Model

The biggest difference between an STA and an MTA threaded apartment is that an MTA apartment can have more than one thread running simultaneously in the same apartment using all the shared data available in the apartment.