Control how data is delivered with ASP.NET
Wednesday, January 18, 2006 04:16 PM
During a recent Web project, I focused on the coding, while a Flash developer handled the user interface. The interface required data delivered from a backend data source and the Flash developer requested text files. This was an unsuitable delivery method since the data varied according to the user.
I convinced him to call an ASP.NET page, where I manipulated the page's content type to text rather than standard HTML. I was surprised when discussing the project with other developers who seemed unaware of this approach. I'll describe the method in detail in this article.
The type of content
When a Web client requests a page, the server informs the client of the type of data it is sending. It is returned via the HTTP content-type header. Most servers are configured to send the content type according to file extension or MIME type.
In Internet Information Services (IIS), you can add new file types via the properties dialog box of the default Web site. The HTTP Headers tab contains a File Types button. You can use it to add new file types. Here is a sampling of some common file types:
- css: A cascading style sheet file.
- doc: A Microsoft Word document.
- html: A standard HTML file.
- jpg: An image file.
- mpg: A video file.
The HTTP protocol does not use the file extension to determine the type of content contained in a file; after all, some files may not have file extensions. Instead, the Web server specifies the correct MIME type using a content-type header when responding to requests.
MIME types identify the type of data contained in a file. MIME types are similar to file extensions, but they are more universally accepted. In our previous list, the MIME type for css content is text/css; text/html is the type for an html file; and image/jpeg is the type for jpg files. While Web servers often use the file extension as previously discussed, a MIME type should always take precedence. The Internet Assigned Numbers Authority (IANA) provides a thorough MIME reference list online.
Now let's examine how to manipulate the content type for an ASP.NET page.
Controlling ASP.NET content
The HttpResponse class controls the various aspects of ASP.NET's response to
HTTP requests. It allows you to easily access the output sent to the client as
well as other attributes. This includes the character set, encoding, and the
content type.
The ContentType property of the HttpResponse class provides access to the MIME type of the response. It allows you to retrieve or set its value, and the value must be a valid MIME type. The C# code sample in Listing A sends data read from the sample SQL Server Northwind database as a text file. Listing B contains the equivalent VB.NET code.
This simple example demonstrates the use of the ContentType property. The code connects to SQL Server and reads all records from the Customers table in the Northwind database. All data is sent to the browser with the ContentType property of the Response object set to the text/plain MIME type to signal text. The code also allows text to be generated by ASP.NET while utilizing ASP.NET programming. That is, you could take advantage of ASP.NET security or other features to control access to the page, making it much more powerful than a generic text file.
While the Response class provides easy access to the ContentType property, you may also specify it via the page directive located at the top of the ASP.NET page. ContentType is a property of the page directive. The following line could be used to signal the type of content sent to the client:
<%@ Page language="c#" ContentType="text/plain" %>
Using this approach, the example code could take the format that's in Listing C.
Listing D demonstrates a shortcut for exporting data to Excel (assuming Excel is installed on the client machine). The code duplicates the previous example, but the content type is set to application/vnd.ms-excel to specify a Microsoft Excel file. Note that tabs are used as separators (/t) as opposed to a comma, and the user is prompted to open the content in Excel.
Odd behavior
Browser behavior does not always follow the rules. While you may specify
text, an image file, or another MIME type as the content for a page, the browser
may choose to treat a file differently. A good example is my recent experience
with IE 6. I tested the output of text in the browser, and IE would often
mistake the text output (specified as text/plain MIME type) as XML and notify me
of errors in the XML. I used Firefox for testing as well, and it handled the
files as planned. This wasn't a big problem for my project since the browser was
only used for testing, but it could hamper a different project. Check out this good discussion of these anomalies.
Deliver the right data
Different projects call for different data and formats. In order to ensure
that you're delivering the right type of data, you should use the HTTP content
type header. You can access this header via the HttpResponse class or using the
ASP.NET page directive. In addition, you can manipulate the content as necessary
using MIME types.
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