ProcessRequest()

            This method is the core of HttpHandler as it does all the processing of an HttpHandler.

IsReusable

            This property specifies whether the HttpHandler can be reused for similar request.

So the HttpHandler implementation should contain implementation of both the members.

Where to use?

I will explain a sample scenario where we can use the HttpHandler in a normal application. Assume that we are developing an application that deals with lots of images and the complication is when we have to display the entire images in the database i.e. in BLOB. To display it in a page we need to construct a new page which fetches the image from database and do a BinaryWrite .i.e. the final code will be like,

  imImage.ImageUrl = "~/ImageHandler.aspx?ImID=100;

Where imImage is image control and ImageHandler.aspx will have the code,

string imageid = context.Request.QueryString["ImID"];

SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["connectionString"].ConnectionString);

connection.Open();

SqlCommand command = new SqlCommand("select Image from Image where ImageID="+imageid, connection);

SqlDataReader dr = command.ExecuteReader();

dr.Read();

context.Response.BinaryWrite((Byte[])dr[0]);

connection.Close();

context.Response.End();

If we consider the above scenario to use an ASPX page to fulfill a single database operation where the other page events are not necessary i.e. the ASPX page will have a series of event starting from Init to Unload which is an overhead to this simple database operation. One can understand the complexity if we are trying to populate more images in a single page using the above approach. It is in this place we can think about using HttpHandler to do the above process. As we know the HttpHandler will be the endpoint for the request as opposed to the above ASPX page. Now the HttpHandler will have only one method ProcessRequest which does the simple operation and thus preventing a series of events. So how does the ProcessRequest method get the access of Response object? If we see the signature of ProcessRequest it will be like,

public void ProcessRequest (HttpContext context)

{

}

The context object will give access to necessary object about the request. It is through which we can do a BinaryWrite using Response object. Other scenarios where we can use HttpHandler is, if we want to export the dataset content to Excel and if we want to have a file with our own extensions like .axd we can use HttpHandler. We have to make appropriate config setting in Web.Config to work,

<httpHandlers>

       <add verb="supported http verbs" path="path" type="namespace.classname, assemblyname" />

<httpHandlers>

Where verb are “GET, POST”, path will be like “*.axd” for the above said example and type will be the actual type of the handler.

Developing a HttpHandler using Visual Studio 2005

Create a new WebApplication project. Right click the solution and click “Add New Item” menu which will open the new items dialog box like,

Select Generic handler and click Add. Now add the following code,

<%@ WebHandler Language="C#" Class="ImageHandler" %>

using System;

using System.Web;

using System.Configuration;

using System.Data.SqlClient;

public class ImageHandler : IHttpHandler {

    public void ProcessRequest (HttpContext context) {

        string imageid = context.Request.QueryString["ImID"];

        SqlConnection connection = new         SqlConnection(ConfigurationManager.ConnectionStrings["connectionString"].ConnectionString);

        connection.Open();

        SqlCommand command = new SqlCommand("select Image from Image where ImageID="+imageid, connection);

        SqlDataReader dr = command.ExecuteReader();

        dr.Read();

        context.Response.BinaryWrite((Byte[])dr[0]);

        connection.Close();

        context.Response.End();             

    }

    public bool IsReusable {

        get {

            return false;

        }

    }

}

To display the image in ASPX page,

  imImage.ImageUrl = "~/ImageHandler.ashx?ImID=100;

Where imImage is image control and ImageHandler.ashx is the HttpHandler.