CodeBehind not used at runtime, make sure that you have given class name correctly and class implemented IHttpHandler interface
sample code:
<%@ WebHandler Language="C#" Class="Handler" %>
using System;
using System.Web;
public class Handler : IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "text/plain";
context.Response.Write("Hello World");
}
public bool IsReusable
{
get
{
return false;
}
}
}