Why are WinForms applications STAThread by default?

c# .net winforms sta
Daniel Peñalba picture Daniel Peñalba · Jan 11, 2011 · Viewed 11.4k times · Source

When you create an empty WinForms application with Visual Studio, the template has the STAThread attribute in the main application class.

I have been reading some docs about it, but I'm not sure if I understood it at all.

Really I have some questions about it:

  1. Why is this attribute added?
  2. What does it mean?
  3. What happens if you remove this attribute?

Answer

Alex Fort picture Alex Fort · Sep 19, 2008

To quote from an MSDN blog,

When the STAThreadAttribute is applied, it changes the apartment state of the current thread to be single threaded. Without getting into a huge discussion about COM and threading, this attribute ensures the communication mechanism between the current thread and other threads that may want to talk to it via COM. When you're using Windows Forms, depending on the feature you're using, it may be using COM interop in order to communicate with operating system components. Good examples of this are the Clipboard and the File Dialogs.

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