B# .NET Blog

Introduction

In my previous blog post on Code 39 barcodes, I've shown you guys how to generate Code 39 barcodes using managed code. Today, we'll take it another step forward and make barcode generation available through an .ashx ASP.NET 2.0 HTTP handler.

About ASP.NET 2.0 Generic Handlers

One of the great things in ASP.NET is the concepts of "generic handlers". Basically you can look at it as just another kind of object that can process HTTP requests, however outside the scope of a page (which targets 'classic' HTML-based output). You might have heard about HTTP handlers too and basically these are the same except for the fact that you can bind an HTTP handler to any file extension (as long as you IIS configuration permits) whileas a generic handler just lives behind the .ashx extension and is directly supported in Visual Studio 2005 web site projects. In the end all handlers implement System.Web.IHttpHandler. Last but not least, you'll be able to host any ASP.NET HTTP Handler directly in IIS 7 too (which I'll blog about later on).

Our online Code39 barcode generator

First of all create a new web site project in Visual Studio 2005:

Next, add a new generic handler called "Code39Generator.ashx" to the project:

This will generate the following piece of code:

<%@ WebHandler Language="C#" Class="Code39Generator" %>

using System;
using System.Web;

public class Code39Generator : IHttpHandler {
   public void ProcessRequest (HttpContext context) {
      context.Response.ContentType = "text/plain";
      context.Response.Write("Hello World");
   }

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

Exactly, "Hello World" made it to the built-in code snippets in Visual Studio 2005. Our task is to create a suitable ProcessRequest method that returns an image with a requested barcode. Requested by means of the querystring that is. The IsReusable property is used by the ASP.NET runtime to find out whether the instance of our handler can be reused for multiple requests. That's fine for us, as we don't have intentions to turn our handler instance to garbage once a request has passed through the pipeline:

   public bool IsReusable {
      get {
         return true;
      }
   }

On to the real stuff now. First of all make sure the Code39 and Code39Settings classes (see previous post) are available in the context of the web site project. You can do that by CTRL-C,V-ing the code to a class file or by referencing a class library project. I've chosen to walk the former path as shown below:

Make sure to declare the classes as follows:

Back to our .ashx file. First let's fix some namespace imports:

Real stuff happens a little further, in the ProcessRequest method which we define like this:

public void ProcessRequest(HttpContext context)
{
   string code = context.Request["code"];
   if (code != null)
   {
      Image img = new Code39(code.ToUpper()).Paint();
      context.Response.ContentType = "image/png";
      byte[] b = GetImageBytes(img);
      context.Response.OutputStream.Write(b, 0, b.Length);
   }
}

[ExternalException (0x80004005): A generic error occurred in GDI+.]
   System.Drawing.Image.Save(Stream stream, ImageCodecInfo encoder, EncoderParameters encoderParams) +568
   System.Drawing.Image.Save(Stream stream, ImageFormat format) +33
   Code39Generator.ProcessRequest(HttpContext context) +157
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +390
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +65