title: C# using statement rejected in one class but accepted in another in same project
body: <p>I have a WPF desktop project targeting .Net 4.5.1.
  Two identical using statements (verified by cut and paste):</p>
  
  <pre><code>using System.Windows.Data;
  using System.Windows.Controls;
  </code></pre>
  
  <p>The definitions are accepted in one class but rejected in another. <strong>The type or namespace 'Data' does not exist in namespace 'System.Windows'</strong> </p>
  
  <ul>
  <li>Both classes are in the same project.</li>
  <li>Both classes are in the same namespace.</li>
  <li>They are in different folders within the project structure.</li>
  <li>I have project references to both PresentationCore and
  PresentationFramework.</li>
  <li>The project is targeting Mixed Platforms/ Any CPU</li>
  </ul>
  
  <p>The one working is:</p>
  
  <pre><code>using System;
  using System.Windows;
  using System.Windows.Input;
  using System.Windows.Data; 
  using System.Windows.Controls; 
  using System.Collections.Generic; 
  using System.Text; 
  using System.Data.SqlClient; 
  using PO.Model; 
  
  namespace PO 
  { 
      public partial class frmPOOrders : Window 
  </code></pre>
  
  <p>while the broken one is:</p>
  
  <pre><code>using System; 
  using System.Windows; 
  using System.Windows.Data; 
  using System.Windows.Controls; 
  using System.Collections.Generic; 
  using System.Text; 
  using System.Windows.Input; 
  using System.Data; 
  using System.ComponentModel; 
  using System.Globalization; 
  using System.Windows.Markup; 
  
  namespace PO 
  { 
      /// See: hardcodet.net/2008/04/wpf-custom-binding-class
      /// &lt;/summary&gt; 
      [MarkupExtensionReturnType(typeof(object))] 
      public abstract class BindingDecoratorBase : MarkupExtension 
  </code></pre>