What is an HttpHandler in ASP.NET

What is an HttpHandler in ASP.NET? Why and how is it used?

In the simplest terms, an ASP.NET HttpHandler is a class that implements the System.Web.IHttpHandler interface.

ASP.NET HTTPHandlers are responsible for intercepting requests made to your ASP.NET web application server. They run as processes in response to a request made to the ASP.NET Site. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page through the page handler.

ASP.NET offers a few default HTTP handlers:

• Page Handler (.aspx): handles Web pages
• User Control Handler (.ascx): handles Web user control pages
• Web Service Handler (.asmx): handles Web service pages
• Trace Handler (trace.axd): handles trace functionality

You can create your own custom HTTP handlers that render custom output to the browser. Typical scenarios for HTTP Handlers in ASP.NET are for example

• delivery of dynamically created images (charts for example) or resized pictures.
• RSS feeds which emit RSS-formated XML

You implement the IHttpHandler interface to create a synchronous handler and the IHttpAsyncHandler interface to create an asynchronous handler. The interfaces require you to implement the ProcessRequest method and the IsReusable property.

The ProcessRequest method handles the actual processing for requests made, while the Boolean IsReusable property specifies whether your handler can be pooled for reuse (to increase performance) or whether a new handler is required for each request.

An HttpHandler (or IHttpHandler) is basically anything that is responsible for serving content. An ASP.NET page (aspx) is a type of handler.

You might write your own, for example, to serve images etc from a database rather than from the web-server itself, or to write a simple POX service (rather than SOAP/WCF/etc)