How do I find if the current executed code is on the UI thread?

Question

I am developing a silverlight application and I have a method that should throw an exception if it's ran on the UI thread since it uses AutoResetEvent.WaitOne() without timeout which causes the UI thread to freeze.
How do I detect if the current thread is the UI thread?

EDIT:
Note that this method resides in a class library and not in a UI component.

Answer 1

You can check if you are in the main UI thread by using Application.Current.Dispatcher (use the CheckAccess() function).

Notice i said main UI thread - this is important, as any thread can have a dispatcher associated with it*, and you can have more than one UI thread. To be more accurate, your question should be

How do I find if the current executed code is a UI thread?

AFAIK this may not be possible with any certainty, as the only real discernable difference (that i have seen) between a background (thread pool) thread and a UI thread is their ApartmentState - UI threads are STA, and the threads available in the thread pool are MTA. This doesn't mean that a STA thread is a UI thread though. (I would be glad for someone to chip in with something definitive here) . Maybe your solution is to look at it the other way - ensure that the AutoResetEvent.WaitOne() calls are only done on threads that are MTA.

_{*just try executing the code this.Dispatcher.CheckAccess() on a background thread and see what happens....}

Edit:

thanks to @drow for pointing out that this should be Silverlight... duh. I will leave the answer above untouched as it is still applicable for WPF. As mentioned by @drow, there is no way to access the ApartmentState of the thread in Silverlight, and the Dispatcher is not available when using System.Threading.Thread.CurrentThread .

So as another option you can check whether Deployment.Current.Dispatcher.CheckAccess() returns false , if it is then you are on a background thread (ie not the main UI thread). I did test this, and it works nicely.