ASP.NET ASHX Handler Tutorial

ASHX handler. Some ASP.NET files are dynamic.

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 return an image from a query string, write XML, or any other data.

Tutorial. First, we review the goal of using ASHX files in the ASP.NET web development framework. We use the ASHX file in a URL, and have it dynamically return content. We will use the query string. The final URLs will look like this.

Url example

?file=name

Getting started. We list the steps to add a new ASHX file. To do this, open your ASP.NET web site. Go to the Website menu and click on the first menu item there, "Add New Item...". This will present the Add New Item dialog box.

Then: Select the "Generic Handler" item, and you will get a new file with some code in it called Handler.ashx.

Autogenerated code. What purpose does the autogenerated code in the ASHX file have? It defines two parts of the IHttpHandler interface. The important part is ProcessRequest(), which will be invoked whenever the Handler.ashx file is requested.

Tip: You should not change the interface inheritance or remove either of the members.

Mappings. 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.

Tip: To do this, use urlMappings. Alternatively you can use more complex RewritePath methods.

Part of Web.config file: XML

<system.web>
    <urlMappings enabled="true">
	<add url="~/Default.aspx" mappedUrl="~/Handler.ashx"/>
    </urlMappings>
    ...

URL mappings. The above Web.config markup will automatically link one URL to another. 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 Example: Redirects

Add example image. 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. We implement an image-based handler.

Modify Handler.ashx. Your handler has two parts. Here we 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 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. You can add any functionality (logging code or referrer logic) to the handler in the C# language.

Also, 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. The code is the same.

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.

URL 1

	      URL: ?file=logo
File query string: logo
     File written: Logo1.png

URL 2

	      URL: ?file=flower
File query string: flower
     File written: Flower1.png

Test query string. Does all this work? Yes, but it is always important to test. Open your browser. And on the path, add query strings like those shown above. What happens is that ASP.NET internally maps the Default.aspx page to your Handler.ashx.

Then: The Handler.ashx file receives the query string and writes 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 detailed initialization and event model. You don't need that for dynamic images, XML, or binary files.

Image Output

Performance. Is there any performance advantage or change to using ASHX files? These files are less complex and they do not involve as many events. They are more streamlined and involve less code, and this is an advantage.

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.

IsReusable: I do not know precisely what the IsReusable property does. Much of the supporting code is not in the ASHX file itself.

But: My reading indicates it can improve performance and decrease memory pressure by not repeatedly destroying the handler.

Choose handlers. Here I want to propose some guidelines about when to choose custom handlers and when to prefer 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.

Note: This will result in greater performance. And the code will be much simpler code to debug.

Summary. We used ASHX custom handlers in an ASP.NET website. This could fill many different important web site functions. We combine urlMappings with query strings on custom handlers to greatly simplify and streamline back-end web site code.

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