When I add an HTTP handler:
With the class:
using System;
usin
I guess you are using a web site project in contrast of web application project. In this case you need to put the code behind file of your handler (Handler.cs) in the special App_Code folder. The markup file (Handler.ashx) may be at the root of your site:
<%@ WebHandler Language="C#" Class="Handler" CodeBehind="Handler.cs" %>
Then you can directly declare your handler in web.config: