multithreading - In WinForms, why can't you update UI controls from other threads?

Question

I'm sure there is a good (or at least decent) reason for this. What is it?

Answer 1

I think this is a brilliant question - and I think there is need of a better answer.

Surely the only reason is that there is something in a framework somewhere that isn't very thread-safe.

That "something" is almost every single instance member on every single control in System.Windows.Forms.

The MSDN documentation for many controls in System.Windows.Forms, if not all of them, say "Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe."

This means that instance members such as TextBox.Text {get; set;} are not reentrant.

Making each of those instance members thread safe could introduce a lot of overhead that most applications do not need. Instead the designers of the .Net framework decided, and I think correctly, that the burden of synchronizing access to forms controls from multiple threads should be put on the programmer.

[Edit]

Although this question only asks "why" here is a link to an article that explains "how":

How to: Make Thread-Safe Calls to Windows Forms Controls on MSDN

http://msdn.microsoft.com/en-us/library/ms171728.aspx