title: System.Windows.Freezable is missing
body: <p>I have got a Class Library project where I keep some interfaces and classes for my WPF Application and RIA Services.</p>
  
  <p>Into the one of class I want to add a member </p>
  
  <pre><code>Brush BackgroundColor { set; get; }
  </code></pre>
  
  <p>or</p>
  
  <pre><code>SolidColorBrush BackgroundColor { set; get; }
  </code></pre>
  
  <p>But I got this error. </p>
  
  <blockquote>
    <p>Error 2   The type 'System.Windows.Freezable' is defined in an assembly
    that is not referenced. You must add a reference to assembly
    'WindowsBase, Version=4.0.0.0, Culture=neutral,
    PublicKeyToken=31bf3856ad364e35'.</p>
  </blockquote>
  
  <p>What's up?</p>
  
  <p>How to fix it if under <em>Add Reference</em> window I cannot see that assembly at all?</p>