The "sender" parameter is a reference to the control instance that  
generated the click.  If you want the Button itself, you can just cast  
that to the Button type, or if all you need is something from the base  
Control class (e.g. the Tag property, commonly used to associate data with  
a particular control), just cast it to the Control type.

You can even just compare the reference to your class variables that  
reference each button (e.g. "button1", "button2", etc.).  IMHO this is the  
least preferable way to handle special-case behavior for a given button in  
a shared handler, but for small numbers of buttons it's not actually all  
that bad.

Finally, it probably goes without saying, but if you have drastically  
different behavior in the handler depending on which button is clicked,  
it's better to just have a different handler for each button.

Pete