I am learning C# 3.5 and I want to know what [STAThread] does in our programs?
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.net-attributes
The STAThreadAttribute marks a thread to use the Single-Threaded COM Apartment if COM is needed. By default, .NET won't initialize COM at all. It's only when COM is needed, like when a COM object or COM Control is created or when drag 'n' drop is needed, that COM is initialized. When that happens, .NET calls the underlying CoInitializeEx function, which takes a flag indicating whether to join the thread to a multi-threaded or single-threaded apartment.
Read more info here (Archived, June 2009)
and
It tells the compiler that you're in a Single Thread Apartment model. This is an evil COM thing, it's usually used for Windows Forms (GUI's) as that uses Win32 for its drawing, which is implemented as STA. If you are using something that's STA model from multiple threads then you get corrupted objects.
This is why you have to invoke onto the Gui from another thread (if you've done any forms coding).
Basically don't worry about it, just accept that Windows GUI threads must be marked as STA otherwise weird stuff happens.
Source: Stackoverflow.com