Question

I’ve noticed in WPF that when trying to update the UI from a background thread (I know that you’re not supposed to do this – just playing around with things) that sometimes it throws an InvalidOperationException, and sometimes it just does nothing. I first noticed this when I was improperly trying to update the UI from a background thread started by an async WCF call (using Begin/End, not the eventing model that automatically marshals to the UI thread).

For example, let’s say I’ve got a simple form with a button and a checkbox. This code will throw an InvalidOperationException (“The calling thread cannot access this object because a different thread owns it.”) every time:

private void button1_Click(object sender, RoutedEventArgs e)
{
    new Thread(() => checkBox1.IsChecked = true).Start();
}

Now, take the same form, and add a bog-standard service reference to a simple WCF service somewhere. Then try this:

private void button1_Click(object sender, RoutedEventArgs e)
{
    var client = new MyServiceClient();
    //Note the use of Begin/End as opposed to the eventing model - Callback
    //will not be called on the UI thread, but a worker thread. I have
    //verified this through the debugger thread list and by checking the 
    //result of Dispatcher.CheckAccess() in the callback.
    client.BeginMyServiceMethod("MyArgument", Callback, null);
}

private void Callback(IAsyncResult result)
{
    //If I call Dispatcher.CheckAccess() here, it returns false,
    //but if I call Dispatcher.VerifyAccess() it does not throw!
    checkBox1.IsChecked = true;  // no exception, no effect
}

My understanding is that a call to Dispatcher.VerifyAccess() in that callback method should throw, as should trying to manipulate anything on checkBox1. Instead, nothing happens – the checkbox in the UI does not become checked, and no exception is thrown. Does anyone know why this is the case?

Answer 1

Is it possible that the Framework code that’s making the call to the callback method is swallowing exceptions? Put a try/catch in the callback to see if exceptions are being thrown. If so, then you know that the Framework is swallowing exceptions.