Question

I want to otherwise block code execution on the main thread while still allowing UI changes to be displayed.

I tried to come up with a simplified example version of what I’m trying to do; and this is the best I could come up with. Obviously it doesn’t demonstrate the behavior I’m wanting or I wouldn’t be posting the question. I just hope it gives some code context to back my poor explanation of the problem I’m hoping to solve.

Within a button click handler on a form I have this:

    private void button2_Click(object sender, EventArgs e)     {         AutoResetEvent autoResetEvent = new AutoResetEvent(false);          new Thread(delegate()          {             // do something that takes a while.             Thread.Sleep(1000);              // Update UI w/BeginInvoke             this.BeginInvoke(new ThreadStart(                 delegate() {                      this.Text = 'Working... 1';                     this.Refresh();                     Thread.Sleep(1000); // gimme a chance to see the new text                 }));              // do something else that takes a while.             Thread.Sleep(1000);              // Update UI w/Invoke             this.Invoke(new ThreadStart(                 delegate() {                     this.Text = 'Working... 2';                     this.Refresh();                     Thread.Sleep(1000); // gimme a chance to see the new text                 }));              // do something else that takes a while.             Thread.Sleep(1000);              autoResetEvent.Set();         }).Start();           // I want the UI to update during this 4 seconds, even though I'm          // blocking the mainthread         if (autoResetEvent.WaitOne(4000, false))         {             this.Text = 'Event Signalled';         }         else         {             this.Text = 'Event Wait Timeout';         }         Thread.Sleep(1000); // gimme a chance to see the new text         this.Refresh();     } 

If I didn’t set a timout on the WaitOne() the app would deadlock on the Invoke() call.

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As to why I’d want to do this, I’ve been tasked with moving one subsystem of an app to do work in a background thread, but still have it block user’s workflow (the main thread) only sometimes and for certain types of work related to that subsystem only.

Answer 1

I went with something I haven’t seen posted yet which is to use MessageQueues.

• The MainThread blocks while waiting for the next message on a queue.
• The background thread posts different types of messages to the MessageQueue.
• Some of the message types signal the MainThread to update UI elements.
• Of course, there is a message to tell the MainThread to stop blocking and waiting for messages.

Seems over the top considering the windows message loop already exists somewhere, but it works.