title: Error: the type or namespace UI doesnt exist in the namespace
body: <p>I need to use System.Web.UI in a class library project. </p>
  
  <pre><code>using System.Web.UI;
  
  namespace OnlinePdViewer
  {
      public class DisplayPd
      {
      }
  }
  </code></pre>
  
  <p>I get compile error when using System.Web.UI statement: </p>
  
  <p>"The type or namespace name 'UI' does not exist in the namespace in the 'System.Web' are you missing an assembly reference?"</p>
  
  <p>I tried to add the reference System.Web, but COM does not include System.Web dll.</p>
  
  <p>Can you help me figure out this? Thank you.. </p>