Controls.Add ArgumentException

When executing this rather innocuous line of code within a System.Windows.Forms.Form:

this.Controls.Add(new Label());

I get this cruel, indescript error:

Note that I get the error no matter what type of Control I try to add (e.g. TextBox, ListView, etc.). Unfortunately, this is the worst type of error because the Exception can't be easily searched via Google.

My hunch at the moment is that this Exception is indicative of something more wrong than an ArgumentException (particularly because I KNOW that the argument is correct). The method that contains this Control.Add function is called by an event triggered by a Timer thread, I suspect that this could be the root of the problem. I found a post in the .NET newsgroups stating that manipulating a Windows Form from another thread is highly hazardous.

Update July 8th, 2005 12:29AM: I found out more information about the underlying problem here. From Code Project,

.NET allows you to call System.Windows.Forms.Control functions only from the thread in which the control was created. To run them from another thread we need to use the Control.Invoke (synchronous call) or Control.BeginInvoke (asynchronous call) functions.

Aha, now we're on the right track. However, note that .NET Compact Framework 1.0 does not support Control.BeginInvoke (although .NET CF 2.0 does). That's fine though. I created my own delegate and tried this on my own -- something really simple like public delegate void MyDelegate(); But then what do you know, I get the exact same System.ArgumentException as before but now when I call MyForm.Invoke(MyDelegate). It took me forever to track down why because it's in very small print in the MSDN API for the Control.Invoke method (and I missed it quite a few times):

Platform Note: In .NET Compact Framework applications, the delegate must be an instance of EventHandler. For an example, see How to: Use a Custom Delegate.

So, you can't use your own delegate, it has to be an instance of EventHandler. I haven't actually tried this yet because it took me roughly two hours to figure this out. I originally found out about this delegrate constraint here:

7.10. Can I create a custom delegate to pass to invoke?

No, in the .NET Compact Framework, only EventHandler methods can be invoked. The proper method for doing this is described here:

public void HandleMe(object o, EventArgs e) {...}
form.Invoke(new EventHandler(form.HandleMe));

Although the following will compile, it will not work properly:

public delegate void MyHandler();
public void HandleMeBadly() {...}
form.Invoke(new MyHandler(form.HandleMeBadly));

In fact, the HandleMeBadly code above does compile, which makes it even more confusing. If you try to run that code, you will get an ArgumentException error. Given what I know now, however, that Exception actually makes sense, as opposed to when I called this.Controls.Add(new Label()) in the non-creation thread.