IHttpHandler

IHttpHandler is an interface defined in the System.Web namespace of the .NET Framework. It represents an object that processes HTTP requests in the ASP.NET pipeline. Implementing this interface allows developers to create custom handlers that respond to specific URL patterns or request types.

The interface declares two members: ProcessRequest(HttpContext context) and IsReusable. ProcessRequest receives an HttpContext object that contains all information about the current HTTP request, including query strings, headers, and the response stream. The implementing class performs logic and writes the response directly to context.Response. IsReusable indicates whether the same handler instance can be reused for multiple requests; returning true allows the framework to pool the handler, improving performance, but the implementation must be thread‑safe.

Handlers are integrated into the ASP.NET request processing sequence after authentication and authorization but before the response is sent to the client. A handler can be mapped to a URL pattern in Web.config through the <system.web/handlers> section, or it can be invoked via the Global.asax Application_BeginRequest event or by using a wildcard mapping.

When writing a handler, developers typically override the DoProcessing logic inside ProcessRequest: read input, perform business operations, and output formatted content such as HTML, JSON, XML, or binary data. Because ProcessRequest runs on a thread from the ASP.NET thread pool, long‑running operations should use asynchronous programming or off‑load work to avoid blocking the thread.

In ASP.NET MVC and Web API these frameworks use controllers and action methods rather than IHttpHandler directly, but underlying the framework the handler infrastructure still processes the HTTP request before routing. Thus, IHttpHandler remains a core component for scenarios that require low‑level request handling, such as custom file download handlers, image or PDF generators, or specialized protocol endpoints.