写HttpHandler与ASP.NET WebForm基本没有什么不同。只是部署的时候需要注意，只在Web.config中注册还不行，需要在Global.asax.cs添加一条ignore规则：

        protected void Application_Start()
        {
            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);

            RouteTable.Routes.IgnoreRoute("{filename}.ashx");
            RouteConfig.RegisterRoutes(RouteTable.Routes);
        }

链接：

http://www.hanselman.com/blog/BackToBasicsDynamicImageGenerationASPNETControllersRoutingIHttpHandlersAndRunAllManagedModulesForAllRequests.aspx

在ASP.NET MVC中支持 HttpHandler