 <p>The features you are trying to use are part of Web API 2. Since you mentioned you're using MVC 4, I expect you will want to upgrade your references to MVC 5 and Web API 2.</p> <p>Find out more about these features at <a href="http://www.strathweb.com/2013/06/ihttpactionresult-new-way-of-creating-responses-in-asp-net-web-api-2/">http://www.strathweb.com/2013/06/ihttpactionresult-new-way-of-creating-responses-in-asp-net-web-api-2/</a></p>