title: The type or namespace name 'ObservableCollection' could not be found
body: <p>I am working with Entity Framework in asp.net c# framework 3.5
  I have generated entity classes using poco generator template.
  But I am getting following error: </p>
  
  <blockquote>
    <p>The type or namespace name <code>ObservableCollection</code> could not be found
    (are you missing a using directive or an assembly reference?)</p>
  </blockquote>
  
  <p>FYI <code>System.Collections.ObjectModel</code> is also added in class.</p>
  
  <p>What could be wrong? How to resolve it?</p>