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http://www.strathweb.com/2013/06/supporting-only-json-in-asp-net-web-api-the-right-way/

public class JsonContentNegotiator : IContentNegotiator
{
    private readonly JsonMediaTypeFormatter _jsonFormatter;
 
    public JsonContentNegotiator(JsonMediaTypeFormatter formatter) 
    {
        _jsonFormatter = formatter;    
    }
 
    public ContentNegotiationResult Negotiate(Type type, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)
    {
        var result = new ContentNegotiationResult(_jsonFormatter, new MediaTypeHeaderValue("application/json"));
        return result;
    }
}

//Now all that’s left to do is to register the new mechanism against your instance of the HttpConfiguration:
var jsonFormatter = new JsonMediaTypeFormatter();
//optional: set serializer settings here
config.Services.Replace(typeof(IContentNegotiator), new JsonContentNegotiator(jsonFormatter));