How to use ICallBack Event Handler

I have 2 drop down lists, on 1st DDL selected index changed event I am filling 2nd DDL.... Its a regular postback of asp.net..

In this scenario I want to use ICallBack event handler....

So how can i use this ???

Regards, Rohit Pundlik

You can google it :

1. Client-Callback Implementation (C#) Example
2. Implementing Client Callbacks Programmatically Without Postbacks in ASP.NET Web Pages
3. The ICallbackEventHandler