“Cross thread operation not valid: Control ### accessed from a thread other than the thread it was created.”
Seriously, I don’t think there’s a single person who’s written UI programs for .NET that has not encountered this error. Simply put, there is only 1 thread which does drawing on the screen, and if you try to change UI elements in a thread that’s not the UI thread, this exception is thrown.
A very common example is a progress bar. Let’s say you’re loading a file which takes a long time to process, so you want to notify the user with a progress bar.
public event EventHandler<IntEventArgs> ValueProcessed;
private void StartProcessing() {
ValueProcessed += ValueProcessedHandler;
Thread t = new Thread(delegate() {
for (int i = 0; i < 1000; ++i) {
// do stuff
ValueProcessed(this, new IntEventArgs(i));
});
t.Start();
}
private void ValueProcessedHandler(object sender, IntEventArgs e) {
_progressBar.Value = e.Value;
}
I left some implementation details out, but you can pretty much infer what IntEventArgs is, etc. Once you try to set Value, it’ll throw an cross-thread exception. A common pattern to solve this problem is this:
private void ValueProcessedHandler(object sender, IntEventArgs e) {
if (this.InvokeRequired) {
this.BeginInvoke(ValueProcessedHandler, sender, e);
} else {
_progressBar.Value = e.Value;
}
}
It gets the job done…but frankly I’m too lazy to do that for every single GUI event handler I write. Taking advantage of anonymous delegates we can write a simple wrapper delegate to do this automatically for us.
static class Program {
public static EventHandler<T> AutoInvoke<T>(EventHandler<T> handler) where T : EventArgs {
return delegate(object sender, T e) {
if (Program.MainForm.InvokeRequired) {
Program.MainForm.BeginInvoke(handler, sender, e);
} else {
handler(sender, e);
}
};
}
}
This assumes that you set Program.MainForm to an instance to a Control, typically, as the name implies, the main form of your application. Now, whenever you assign your event handlers, you can simply do this:
ValueProcessed += Program.AutoInvoke<IntEventArgs>(ValueProcessedHandler);
Pretty neat!
BTW, just a word of warning, if you plan on using the same thing to unsubscribe an event with -=, it’s not going to work. Unfortunately, calling the AutoInvoke method twice on the same input delegate returns 2 results where .Equals and == will return false. To get around this you can use a Dictionary to cache the input delegate.