Recipe 17.1: Creating a Reusable Image Handler

Problem

You want to create a reusable assembly that retrieves image data from a database and processes it before sending it to a browser.

Related Reading ASP.NET Cookbook The Ultimate ASP.NET Code Sourcebook By Michael A. Kittel, Geoffrey T. LeBlond

Solution

Create an HTTP handler to read the image data from the database and send it to the browser.

To implement a custom, reusable HTTP handler:

1. Create a separate Class Library project in Visual Studio.

2. Create a class in the project that implements the IHttpHandler interface and place code to handle the request in the ProcessRequest method.

3. Compile the project as an assembly and place the assembly in the bin directory of your web project.

4. Add an <httpHandlers> element to the web.config file in your web project referencing your custom HTTP handler.

5. Reference the URL of the HTTP handler in your application.

Example 17-3 and Example 17-4 show the VB and C# class files we've written to implement an image handler as an HTTP handler. Example 17-5 through Example 17-7 show the .aspx file and VB and C# code-behind files for our application that demonstrates the use of the HTTP handler.

Discussion

HTTP handlers are simply classes that implement the IHttpHandler interface. Implementing the IHttpHandler interface requires the implementation of two methods: IsReusable and ProcessRequest. IsReusable is a property that explicitly returns a Boolean value that indicates whether the HTTP handler can be reused by other HTTP requests. For synchronous handlers, like our example, the property should always return false so the handler is not pooled (kept in memory). The ProcessRequest method is where the actual work is performed and you should place code that processes the requests here.

To create a reusable HTTP handler, you need to eliminate all application-specific code from the class. You must also compile the class as a separate .NET assembly and place the assembly in the bin directory of each application that uses it.

To create an assembly that contains only the handler code, you need to create a separate Class Library project in Visual Studio. In our example, we have named the project VBImageHandler (or CSImageHandler for C#) resulting in an assembly that has the same name as the project. We then compile the assembly, place it in the bin directory of the web project, and add a reference to the assembly.

TIP: When you create a new "Class Library" project for your HTTP handler, you will need to add a reference to the System.Web assembly. This is required because the IHttpHandler interface and HttpContext class used by the HTTP handler are defined in the System.Web assembly.

In our example that demonstrates this solution, we have stored GIF images in a database that can be retrieved and displayed in a browser using our HTTP handler just as if they were standard image files. To demonstrate the HTTP handler, we created an ASP.NET page that contains a DropDownList, a View button, and an HTML img tag. The DropDownList displays the descriptions of the images stored in the database. When you make a selection from the list and click the View button, the src attribute for the img tag is set to the URL of our HTTP handler with the ID of the image in the URL. When the page is then displayed, the browser requests the image from our HTTP handler, which retrieves the ID of the requested image from the URL, reads the data from the database for the image, and then streams the image data to the browser. Figure 17-1 shows the output of the page used to test our HTTP handler.

The image handler implemented in our example needs several pieces of data to retrieve an image from the database. These include the following:

• The connection string for the database

• The name of the table containing the image data

• The name of the column that uniquely identifies an image (the primary key)

• The name of the column containing the image data

• A unique identifier (ID) for the image that is to be displayed

To be reusable, none of this data can be coded directly into the handler. To get around this problem in our example, we declare four public constants in the image handler class that we can use to specify the names of the variables in Application scope that contain the database information. In addition, the ID of the image that is to be downloaded will be passed in the URL used to access the handler (described later).

The application variables defined by the constants are initialized in the Application_Start method of the global.asax.vb (VB) or global.asax.cs (C#) class, which is executed when an application is first started. If you initialize your application variables in the Application_Start method, they will always be available when HTTP requests are processed. The code to implement this approach is in Example 17-1 and Example 17-2.

To create the image handler, you next need to create a class that implements IHttpHandler and its two methods: IsReusable and ProcessRequest. Add the code to process requests made to the handler to the ProcessRequest method. As mentioned, IsReusable is a property that returns a Boolean value indicating whether the HTTP handler can be reused by other HTTP requests. Because our example is a synchronous handler, the property returns false so the handler is not pooled (kept in memory).

In our example, the first step in processing a request for an image is to get the ID of the requested image from the URL that is being processed by the handler.

Next, a connection to the database needs to be opened. The connection string is obtained from an Application scope variable defined by the APP_CONNECTION_STR constant shown in Example 17-1(VB) and Example 17-2 (C#). The name of the database table along with the columns containing the unique identifier and the image data are also obtained from the Application scope variables described earlier. These are then used to create the SQL statement required to read the image data from the database.

The next step in our example is to read the image data from the database using the ExecuteScalar method of the command object. The ExecuteScalar method returns a generic Object, so the return value must be cast to the type of data stored in the database. In this case it must be cast to a byte array.

The image data stored in the database for our example is in the GIF format, so the content type is set to "image/GIF" to inform the browser of the type of data being sent. After setting the content type, the image data is written to the Response object using the BinaryWrite method.

TIP: If your image is of another type, you will need to set the ContentType accordingly. Other choices for images include "image/jpeg", "images/tiff", and "images/png".

In order to use the handler, we next have to add information to the <httpHandlers> element of the web.config file of the application to tell ASP.NET which URL requests it should route to our custom image handler. You insert this information using an add element and its attributes. The verb attribute defines the types of requests that are routed to the HTTP handler. The allowable values are *, GET, HEAD, and POST. The value * is a wildcard that specifies that all request types are to be routed to the handler.

The path attribute defines the URL(s) that are to be processed by the HTTP handler. The path can be set to a single URL, or to a less specific value such as "*.images" to have the HTTP handler process all requests for URLs with an images extension. In our example, we are setting the path to a specific URL (ImageHandlerVB.aspx).

TIP: IIS routes requests with the extensions .asax, .ascx, .ashx, .asmx, .aspx, .axd, .config, .cs, .csproj, .lic, .rem, .resources, .resx, .soap, .vb, .vbproj, .vsdisco, and .webinfo to ASP.NET for processing.

To use an HTTP handler for requests with other extensions, IIS must be configured to send the requests with the desired extensions to the aspnet_isapi.dll.

The type attribute defines the name of the assembly and class within the assembly that will process the request in the format type="class name, assembly". The class name must be identified by its full namespace. Here is the code necessary to add a reference to the image handler to an application web.config file:

<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="ImageHandlerVB.aspx" 
           type="ASPNetCookbook.VBExamples.HttpHandlers.ImageHandlerVB, 
                VBImageHandler" />
    </httpHandlers>
  </system.web>
</configuration>


<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="ImageHandlerCS.aspx" 
           type="ASPNetCookbook.CSExamples.HttpHandlers.ImageHandlerCS, 
                CSImageHandler" />
    </httpHandlers>
  </system.web>
</configuration>

To use the HTTP handler to retrieve images from the database, we need to set the src attribute of image tags that will use the HTTP handler to the name of the HTTP handler defined in the path attribute of the entry added to web.config, passing the ID of the desired image in the URL. In our example, the src attribute of an img tag is set in the view image button click event of the test page code-behind. A sample URL is shown here:

src="ImageHandlerVB.aspx?ImageID=5"

TIP: The HTTP handler does not have to be implemented in the same language as the application. The C# image handler can be used in VB projects or vice versa.