Custom Web API Message Formatter

ASP.NET Web API supports multiple message formats. The framework uses the “Accept” request header from the client to determine the message format. If you visit a Web API URL with internet explorer, you will get a JSON message by default; if you use Chrome, which sends “application/xml;” in the accept header, you will get a XML message.

Message format is an extensivity point in Web API. There is a pretty good example on ASP.NET web site. Recently, when I worked on my SkyLinq open source cloud analytic project, I needed a way to expose data to the client in a very lean manner. I would like it to leaner than both XML and JSON, so I used csv. The example provided on the ASP.NET website is actually a CSV example. The article was lastly updated on March 8, 2012. Since ASP.NET is rapidly advancing, I found that I need to make the following modifications to make the example working correctly with Web API 2.0.

1. CanReadType is now a public function instead of a proetected function.
2. The signature of WriteToStream has changed. The current signature is

   

3. When addition the Media Formatter to the Configuration, I have to insert my custom formatter into the formatters collection as the first. This is because the JSON Formatter actually recognizes the “application/csv” header and send JSON back instead. Example below: