How to create a method in a User control

Hi. I created a user control which has a textbox and a combobox. Depending on the state of the screen (navigation, creation or editing) one of them is visible and the other is invisible. This user control works with a database. Now what I need is to create a method for this component which should work the same way as the SelectedItemChanged of the combobox. But I don't know how to do this.

Can anyone help me?

Thanks.