title: Can't add System.Web.Extensions.dll to References
body: <p>I can't 'System.Web.Extensions.dll' to my References. It is not in the .NET Reference List, so I had to browse to it in:</p>
  
  <pre><code>C:\Program Files\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0
  </code></pre>
  
  <p>But even after that a small error icon is displayed in the library name.</p>
  
  <p>I am trying to use</p>
  
  <pre><code>using System.Web.Script.Serialization;
  </code></pre>
  
  <p>to serialize/deserialize JSON for my application.</p>
  
  <p>What is the problem and how can I fix it ... ?</p>