using ICallbackeventhandler to call code behind functions form javascript in c#

Question

When I use the  ICallbackeventhandler I get an error that it is not implemented by my .aspx page while the downloaded files work in the same.Could any one point out the reason.
Just add name space System.web.UI.Icallbackhandler in your c# code in aspx.cs file and the page wont work.
I t is meant to call code behind  methods from javascript.
Thanks,
Roopangee