Tuesday, July 17, 2012

ASHX file - Web handler file


ASHX file - Web handler file

 
A web handler file works just like an aspx file except you are one step back away from the messy browser level where HTML and C# mix. One reason you would write an .ashx file instead of an .aspx file is that your output is not going to a browser but to an xml-consuming client of some kind. Working with .ashx keeps you away from all the browser technology you don't need in this case. Notice that you have to include the IsReusable property.
<% @ webhandler language="C#" class="AverageHandler" %> 

using System; 
using System.Web; 

public class AverageHandler : IHttpHandler 

   public bool IsReusable 
   { get { return true; } } 
   public void ProcessRequest(HttpContext ctx) 
   { 
       ctx.Response.Write("hello"); 
   } 
}
 

ASP.NET ASHX Handler

Some ASP.NET files are dynamically generated. They are generated with C# code or disk resources. These files do not require web forms. Instead, an ASHX generic handler is ideal. It can dynamically return an image from a query string, write XML, or any other data.
ASHX handler in ASP.NET
This C# tutorial uses the ASHX file format in the ASP.NET framework.

Introduction

First, we review the goal in using ASHX files in the ASP.NET web development framework. What we will be able to do is use the ASHX file in a URL, and have it return content dynamically. We will use the query string, and the final URLs will look like this:
http://www.dotnetperls.com/?file=name
Getting started. This part lists the steps you can take to add a new ASHX file. To do this, open your ASP.NET web site. Go to the Website menu and click on the very first menu item there, "Add New Item...". This will present the Add New Item dialog box. Select the "Generic Handler" item, and you will get a new file with some code in it called Handler.ashx.

Autogenerated code

Here we note what the autogenerated code in the ASHX file does. It defines two parts of the IHttpHandler interface. The important part is ProcessRequest(), which will be invoked whenever the Handler.ashx file is requested or pointed to. You shouldn't modify the interface inheritance or remove either of the members.

urlMappings

It is often desirable to map an older URL or path to your new ASHX file. For backwards compatibility and for search engine optimization, you will probably want the new handler to take over an old URL in your site. To do this, use urlMappings; alternatively, you can use more complex RewritePath methods.
Part of Web.config file [XML]


    
 
    
    ...
urlMappings screenshot of XML
URL mappings. The above Web.config markup will automatically link one URL to another. Now, when the Default.aspx page is requested, your Handler.ashx file will take over. This means you can map the default page in a directory to your handler.
urlMappings for Redirects

Add example image

Here we mention what you can do with the ASHX file involving images. Find your favorite image on your disk or on the Internet and add it to your website project. For my example, the image I chose was "Flower1.png". Next we will use this image in the Handler.ashx file.

Modify Handler.ashx

Your handler has two parts, and here we must modify the ProcessRequest() method. We can change the ContentType of the file and the Response content. Modify your Handler.ashx to be similar to the following, with your image ContentType and file name.
ASHX code-behind file [C#]

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

using System;
using System.Web;

public class Handler : IHttpHandler {

    public void ProcessRequest (HttpContext context) {
 // Comment out these lines first:
 // context.Response.ContentType = "text/plain";
 // context.Response.Write("Hello World");

 context.Response.ContentType = "image/png";
 context.Response.WriteFile("~/Flower1.png");
    }

    public bool IsReusable {
 get {
     return false;
 }
    }
}

Test handler

Here we test the new configuration and ASHX file on the local machine. Now click the green arrow to run your website on the development server. You should see the image in your browser. This is the result of writing the image to the response in the handler.

Add functionality

The example here so far is relatively useless. All it does is allow us to pipe an image through an ASHX handler. Note that you can add any logging code or referrer logic to the handler in the C# language. Developers commonly need to use the QueryString collection on the Request. You can use the Request.QueryString in the Handler just like you would on any ASPX web form page.
ASHX modified code-behind [C#]

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

using System;
using System.Web;

public class Handler : IHttpHandler {

    public void ProcessRequest (HttpContext context) {

 HttpResponse r = context.Response;
 r.ContentType = "image/png";
 //
 // Write the requested image
 //
 string file = context.Request.QueryString["file"];
 if (file == "logo")
 {
     r.WriteFile("Logo1.png");
 }
 else
 {
     r.WriteFile("Flower1.png");
 }
    }

    public bool IsReusable {
 get {
     return false;
 }
    }
}
What this does. The above code receives requests and then returns a different file based on the QueryString collection value. It will return one of two images from the two query strings. The strings it returns are shown next here.
URL = http://www.dotnetperls.com/?file=logo

File query string: logo
     File written: Logo1.png

URL = http://www.dotnetperls.com/?file=flower

File query string: flower
     File written: Flower1.png

Test query string

Does all this really work? Yes, it does, but it is always important to test it. Open your browser and on the path add query strings like shown in the above table. What happens is that ASP.NET internally maps the Default.aspx page to your Handler.ashx. Then, it receives the query string and write the appropriate file.

Uses

The code here could be used as a hit tracker that counts visitors and logs referrers. This could provide a more accurate visit count than server logs, because of browser and bot differences.
Handlers versus web pages. ASP.NET web forms inherit from the Page class. This provides them with many events and a very detailed initialization and event model. You don't need that for dynamic images, XML, or binary files.

Performance

You are likely wondering if there is any performance advantage or change to using ASHX files. ASHX files are less complex and they do not involve as many events. As you can imagine firing more than ten events each time a request is handled is a fair amount more expensive than only firing one event. Therefore, there will be some performance advantage to using ASHX files where possible.

Choose handlers

Here I want to propose some guidelines about when to use custom handlers and when to use ASPX web form pages. Handlers are better for binary data, and web forms are best for rapid development.
Use Web Forms...

If you have simple HTML pages
ASP.NET custom controls
Simple dynamic pages

Use handlers...

If you have binary files
Dynamic image views
Performance-critical web pages
XML files
Minimal web pages

Control trees

In the ASP.NET Framework, web forms use a concept called control trees where the parts of web pages are stored in an object model. Use custom handlers when you do not require the custom control tree or the whole HTML web form framework. This will result in greater performance and much simpler code to debug.

IsReusable property

I do not know precisely what the IsReusable property does in ASP.NET, as much of the supporting code is not in the ASHX file itself. My reading indicates that it can improve performance and decrease memory pressure by not destroying the handler repeatedly.

Summary

We saw how you can use ASHX custom handlers in your ASP.NET website. This could be modified to fill in many different important web site functions. Combine urlMappings with query strings on custom handlers to greatly simplify and streamline your back-end web site code.

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