Why can’t I immediately handle a selection of a Drop-Down list?

I specified a Drop-Down list as Auto Post-back, which causes the Initial event handler for the web page to be processed again with each selection from the list.  But, I also created an event handler routine in C# defined as: public void DropDownList1_SelectedIndexChanged(object sender, EventArgs e), which is never executed when a selection is made in the Drop-Down list.  What do I have to do in order to have this routine be executed whenever a selection is made?
 
Maurice