Quick Question about creating my own control/extender

Would like to know that too

Have you already gone throughhttp://ajax.asp.net/ajaxtoolkit/Walkthrough/CreatingNewExtender.aspx?

That's essentially what I was asking for. Sorry I didn't find it on my own. As that page seems to state, you need to have similar or identical properties in both javascript and server side code. This means if I were to be writing an AJAX validation extender I would need to write validation functions in both javascript and server code?

I've followed the tutorial posted above on how to create a sample AJAX extender. Mostly all of it makes sense, and I was able to follow all of the steps without much trouble. However when I added the onkeyup method I started getting javascript errors during runtime and no AJAX functionality. The error I am getting is:

Line: 5909
Char: 12
Error: Sys.ArguementUndefinedException: Value cannot be undefined.
Parameter name: id
Code: 0

Any suggestions about what to do, or what that error might mean?

Without seeing your code, all I can tell you is you're probably calling a function that takes a parameter called "id" and passing in undefined. :-)