Question,
When using BeginInvoke (on a From to marshal to the UI thread) with the
EventHandler delegate I see some strange behaviour. The problem is that I
call the UpdateTextBox method with a class derived from EventArgs ( i.e.
MyEventArgs) and when the BeginInvoke is called and the UpdateTextBox
methode is call on the UI thread the parameter e (EventArgs) does not
contain the derived MyEventArgs object but a EventArgs object.
The strange thing is that if I create a delegate with the same signature as
the EventHandler ( MyEventHandler) everything works as expected. (See
comments in the code below)
Can anyone explane this to me?
Sandor
(Methods below are implemented on a Form derived class)
private void button1_Click(object sender, System.EventArgs e)
{
Thread t = new Thread(new ThreadStart(OnThreadStart));
t.Start();
}
private void OnThreadStart()
{
UpdateTextBox(null, new MyEventArgs(DateTime.Now.ToLongTimeString()));
}
private void UpdateTextBox(object sender, EventArgs e)
{
if(InvokeRequired)
{
// When using MyEventHandler everything works as expected.
//MyEventHandler h = new MyEventHandler(UpdateTextBox);
EventHandler h = new EventHandler(UpdateTextBox);
BeginInvoke(h, new object[] {sender, e});
}
else
{
// This cast will fail !!!!!!!!!!!!!!!!!!!!!!
MyEventArgs args = (MyEventArgs)e;
textBox1.Text = string.Format("Current time: {0}", args.CurrentTime);
}
}
public class MyEventArgs : EventArgs
{
public readonly string CurrentTime;
public MyEventArgs(string currentTime)
{
CurrentTime = currentTime;
}
}
delegate void MyEventHandler(object sender, EventArgs e);
