Question

I’m writing a little chat app, and I have this event handler:

void o_Typing(object sender, EventArgs e)
{
    MessageBox.Show("Fired!");
    this.Text = "Fired!";
}

o_Typing is a method in a class derived from TabPage. Basically, I want each conversation to have it’s own tab.

The event handlers are fired by my Chat object, which is running in another thread. I have 1 thread for UI, and another thread for each Chat conversation (to keep polling the server for new data)

When the event is fired, the MessageBox pops up, but the Tab caption doesn’t change. After the event has fired once, it never fires again, leading me to believe that the event is being called in the worker thread, although it is defined in the UI thread.

How can I get my events to be called from the worker thread, and use Invoke() to get them to execute on the UI thread?

Answer 1

There are two options:

1) Make the event handlers thread-safe: use Control.Invoke/BeginInvoke in any event handler which needs to talk to the UI thread.

2) Make the worker thread marshal back to the UI thread before raising the event – in other words, use Control.Invoke as part of the process of raising the event, so that the event handlers will all be called in the UI thread. Depending on how your app is structured, you may not want your event-raising component to know about the UI explicitly – but when it’s being constructed you can pass in an ISynchronizeInvoke (which Control implements) and your component can use that to raise its events on the right thread. Of course, that only works (simply, anyway) if every event handler is happy to run on the same thread – but that will often be the case. You’d write something like:

protected void OnFoo(EventArgs args)
{
    if (sync != null && sync.InvokeRequired)
    {
        sync.Invoke((Action) delegate { OnFoo(args) }, null);
        return;
    }
    EventHandler handler = Foo; // Where Foo is the event name
    if (handler != null)
    {
        handler (this, args);
    }
}