title: Can't access System.Windows namespace
body: <p>I have a class that I want to extend the WPF Window class. I cant find the class to extend. I think it's in the <code>System.Windows</code> namespace, but I have no access to it. Can anyone help. </p>
  
  <p>Please let me know if you require anymore information.</p>
  
  <p>I am building the project as a class library.</p>
  
  <p>Current references are:</p>
  
  <pre><code>Microsoft.CSharp
  System
  System.Core
  System.Data
  System.Data.DataSetExtentions
  System.Xml
  System.Xml.Linq
  </code></pre>
  
  <p>I am trying to extend the <code>System.Windows.Window</code> class, but when I type <code>System.</code>, there is no <code>.Windows</code>.</p>