title: Json does not exist in the namespace System
body: <p>In this tutorial: <a href="http://www.asp.net/web-api/videos/getting-started/custom-validation" rel="noreferrer">http://www.asp.net/web-api/videos/getting-started/custom-validation</a> Jon uses</p>
  
  <pre><code>dynamic error = new JsonObject();
  </code></pre>
  
  <p>with</p>
  
  <pre><code>using System.Json;
  </code></pre>
  
  <p>I guess it's the JsonObject here: <a href="http://msdn.microsoft.com/en-us/library/system.json.jsonobject(v=vs.110).aspx" rel="noreferrer">http://msdn.microsoft.com/en-us/library/system.json.jsonobject(v=vs.110).aspx</a> located in: </p>
  
  <ul>
  <li>Namespace:  System.Json</li>
  <li>Assembly:  System.Runtime.Serialization (in System.Runtime.Serialization.dll)</li>
  </ul>
  
  <p>I've added System.Runtime.Serialization a reference but still cannot find System.Json.</p>
  
  <p>Am I reading the Microsoft docs wrong? (I'm using .NET 4.5)</p>