[How Do I:] Create an ASP.NET AJAX Extender from Scratch?
by Chris Pels
Learn how to create an ASP.NET AJAX extender for a standard ASP.NET server control. We are shown how to add server-side properties and client-side JavaScript to extend the behavior of the standard TextBox control, though the same approach can be applied to any other server control. For further help in creating an ASP.NET AJAX extender, please refer to the Creating a New Extender walkthrough.