A couple of questions:
1. Why? (as others asked :-)
2. In a dropdown combo or dropdown list?
3. When the list is dropped open? or when it is closed?

Depending on what you are trying to achieve, it might work better to implement a textbox over a dropdown. Making on or the other invisible depending, of course, on what the user is doing.

>What is the best way of implementing a Click and a DblClick in a Combobox.