Deadlock when invoking the UI thread from a worker thread

The Main thread doesn't look idle. Your screen shot shows it current location at ECM.Program.Main. That can't be correct, if it is idle then it is inside Application.Run(), pumping the message loop. Which is required for Invoke() to complete.

Double-click the main thread and switch to the Call Stack window to find out what it is really doing.

Have you tried using BeginInvoke instead of Invoke? BeginInvoke is asynchronous.

You are correct. The Main Thread is the entry point to the application which is normally the location of the call to Application.Run which gets the message loop going. So that should be the UI thread unless you have done something out of the ordinary in regards to the message loop which is unlikely.

In the Thread window you can right click on the Main Thread and select Switch to change the debugging context to that thread. The Call Stack window will then show the location of the current executing method.

If your worker thread really is blocked on the Control.Invoke call and the UI thread is idle as you claim then the problem could be with the execution of the instructions within the delegate that is being marshaled or the message loop as not yet been started. The later seems plausible since your screen shows the location of the Main Thread as Main.