Question

I have a C# Windows Form application that contains a menu with this event:

private void createMenuItem_Click(object sender, EventArgs e)
    {
        canvas.Layer.RemoveAllChildren();
        canvas.Controls.Clear();
        createDock();
    }

I would like to provide the user with the opportunity to fire this event through another menu option that pulls up a timer.

My timer looks like this:

private void transfer_timer()
 {
  System.Timers.Timer Clock = new System.Timers.Timer();
  Clock.Elapsed += new ElapsedEventHandler(createMenuItem_Click); 
  Clock.Interval = timer_interval;
  Clock.Start();
 }

When I do this the resulting error message is:

createDock – Cross-thread operation not valid: Control ” accessed from a thread other than the thread it was created on.

What am I doing wrong?

Answer 1

Don’t do that. 🙂 Factor it out more like this …

private void createMenuItem_Click(object sender, EventArgs e)
{
  DoCanvasWork(); // pick a good name :)        
}

private void transfer_timer() {
  System.Timers.Timer Clock = new System.Timers.Timer();
  Clock.Elapsed += new ElapsedEventHandler(Clock_Tick);
  Clock.Interval = timer_interval;  Clock.Start(); 
}

private void Clock_Tick( object sender, EventArgs e )
{
  BeginInvoke( new MethodInvoker(DoCanvasWork) );
}

private void DoCanvasWork()
{
  canvas.Layer.RemoveAllChildren();
  canvas.Controls.Clear();
  createDock();
}

That will give you much more clear and much more maintainable code. Your timer is not selecting a menu, it’s doing the work that’s the same as if the menu had been selected.

Per comment: You’re welcome. Glad to help! Here’s a good primer on cross thread marshaling (short read, good examples). But, basically BeginInvoke is taking a delegate (the MethodInvoker) and executing it asynchronously on the thread that the control was created on. In this case, it’s a non-blocking, fire and forget message across threads (i.e. I don’t wait for a return and I don’t call EndInvoke to get a return). If you really want to get in-depth with this, Chris Sells book on Winforms is a great resource.