Question

How am I supposed to import my own namespace into a generic handler (ashx file) ?

— edit: Website not a web application

Answer 1

The ashx file itself should contain code like:

<%@ WebHandler Language="C#" Class="MyProject.MyHandlerClass" %>

And then your project will also contain a class file with that namespace and class:

namespace MyProject
{
    public class MyHandlerClass : IHttpHandler
    {
         public void ProcessRequest(HttpContext context) { ... }
    }
}