Question

hi all
My friend says “in c#, every event handler is bound to a method which its name is On….”
I mean, for example, SelectedIndexChanged of a combobox is bound to a method named OnSelectedIndexChanged.

My question is: How can I see it?

Please tell me more about that if its possible.

Thank you

Answer 1

The methods that start with “On….” are used as a convention. These methods are not “bound” in any way with an event nor an event is “bound” to them (taking into consideration the generally accepted definition/procedure of registering for an event).

What they actually do is raise the event suggested by the words following the “On” prefix.

For instance:

protected void OnSelectedIndexChanged(EventArgs e)
{
   if(SelectedIndexChanged!= null)
      SelectedIndexChanged(this, e);
}

where SelectedIndexChanged event is defined somewhere like this:

public event EventHandler SelectedIndexChanged;

The convention and .net practices state that for each event you should have a overridable protected method that raises the event.

In a derived class you could do something like that:

protected override void OnSelectedIndexChanged(EventArgs e)
{
   base.OnSelectedIndexChanged(e);

   // your custom code here
}

NOTE: To be absolutely correct, this event and others may use different derived types from the EventArgs class but the concept is the same.