How do I make event callbacks into my win forms thread safe?

To simplify Simon's code a bit, you could use the built in generic Action delegate. It saves peppering your code with a bunch of delegate types you don't really need. Also, in .NET 3.5 they added a params parameter to the Invoke method so you don't have to define a temporary array.

void SomethingHappened(object sender, EventArgs ea)
{
   if (InvokeRequired)
   {
      Invoke(new Action<object, EventArgs>(SomethingHappened), sender, ea);
      return;
   }

   textBox1.Text = "Something happened";
}

Here are the salient points:

1. You can't make UI control calls from a different thread than the one they were created on (the form's thread).
2. Delegate invocations (ie, event hooks) are triggered on the same thread as the object that is firing the event.

So, if you have a separate "engine" thread doing some work and have some UI watching for state changes which can be reflected in the UI (such as a progress bar or whatever), you have a problem. The engine fire's an object changed event which has been hooked by the Form. But the callback delegate that the Form registered with the engine gets called on the engine's thread… not on the Form's thread. And so you can't update any controls from that callback. Doh!

BeginInvoke comes to the rescue. Just use this simple coding model in all your callback methods and you can be sure that things are going to be okay:

private delegate void EventArgsDelegate(object sender, EventArgs ea);

void SomethingHappened(object sender, EventArgs ea)
{
   //
   // Make sure this callback is on the correct thread
   //
   if (this.InvokeRequired)
   {
      this.Invoke(new EventArgsDelegate(SomethingHappened), new object[] { sender, ea });
      return;
   }

   //
   // Do something with the event such as update a control
   //
   textBox1.Text = "Something happened";
}

It's quite simple really.

1. Use InvokeRequired to find out if this callback happened on the correct thread.
2. If not, then reinvoke the callback on the correct thread with the same parameters. You can reinvoke a method by using the Invoke (blocking) or BeginInvoke (non-blocking) methods.
3. The next time the function is called, InvokeRequired returns false because we are now on the correct thread and everybody is happy.

This is a very compact way of addressing this problem and making your Forms safe from multi-threaded event callbacks.