Currently, the default response type of for Web API requests is XML. By the time it comes out of beta, it will be the default (source - this is mentioned by Hanselman towards the end of the post). However, if you want to activate this right now, how should you do it?

Two steps:

1. Set a JSON formatter as the first Formatter in the Web API Config Formatters collection
2. Set “text/html” as an accepted media type for this formatter

WebAPI includes a JSON Serializer by default: DataContractJsonSerializer. However, no one wants to use it, and for good reason: lots of issues with different types, slow performance, bad date formatting and more.

Thankfully, WebAPI allows you to switch customize the data formatters used for different content types. Bloggers have recommended a number of different approaches. What seems to be the most promising is Henrik Nielsen’s JsonNetFormatter (using Json.NET to handle the JSON serialization) combined with a fix for a DateTime serialization issue (Hanselman also implies that this will be the default in post-beta WebAPI).

After you add the code for the JsonNetFormatter, you can set this up as the default Json data formatter by doing the following:

protected void Application_Start()
{
    ...

    JsonSerializerSettings serializerSettings = new JsonSerializerSettings();
    serializerSettings.Converters.Add(new IsoDateTimeConverter());
    GlobalConfiguration.Configuration.Formatters.Insert(0, new JsonNetFormatter(serializerSettings));

    ...
}

Once you have done this, any request that comes in to the API that asks for json (setting a header to accept content of type application/json, utilizing the facility in WebAPI for content negotiation) will receive Json content formatter using the JsonNetFormatter class. However, if you want to just test this out in your browser, you will still get XML content. This is because a plan request from your browser is for type text/html, which translates to xml in the Web API universe. Though the Json will be returned automatically if you explicitly request json content (or if you use a function that requests this content type, like the $.ajax function in jQuery), if you want to test out the json in your browser, you will be out of luck using the standard configuration.

To get around this, you need to set the JsonNetFormatter to support the “text/html” media type. This will allow it to respond to requests made from the browser (and since the JsonNetFormatter is now the first Formatter in the Formatters collection, it will be used by default). You can do this as follows:

protected void Application_Start()
{
    ...

    JsonSerializerSettings serializerSettings = new JsonSerializerSettings();
    serializerSettings.Converters.Add(new IsoDateTimeConverter());
    var jsonFormatter = new JsonNetFormatter(serializerSettings);
    jsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));
    GlobalConfiguration.Configuration.Formatters.Insert(0, jsonFormatter);

    ...
}