title: The property does not exist in XML namespace
body: <p>I'm currently working on enabling drag-and-drop support for our company app. <br>
  The error I keep getting seems weird to me. <br>
  What this says is that </p>
  
  <pre><code>The property 'DragDropHelper.IsDragSource' does not exist in XML namespace 'clr-namespace:DragAndDrop;assembly=DragAndDrop'. Line 61 Position 83
  </code></pre>
  
  <p>The property is an attached property in the class I found in the Internet in the samples and modified it a bit. Here is property declaration:</p>
  
  <pre><code>namespace DragAndDrop {
  public class DragDropHelper
  {
      public static readonly DependencyProperty IsDragSourceProperty =    
          DependencyProperty.RegisterAttached("IsDragSource", typeof (bool), typeof (DragDropHelper),
                                              new UIPropertyMetadata(false, IsDragSourceChanged));
  
      public static bool GetIsDragSource(DependencyObject obj)
      {
          return (bool) obj.GetValue(IsDragSourceProperty);
      }
  
      public static void SetIsDragSource(DependencyObject obj, bool value)
      {
          obj.SetValue(IsDragSourceProperty, value);
      }
  </code></pre>
  
  <p>It seems to me that attached property is completely valid, isn't it? 
  This DragDropHelper is included into a class library, that I reference from the main client app.
  When I try to set the value of the property in a client app:</p>
  
  <pre><code> &lt;ListView x:Uid="list" x:Name="CurrentFolderItemsControl" drag:DragDropHelper.IsDragSource="true" /&gt;
  </code></pre>
  
  <p>VS2010 says that property doesn't exist in XML namespace. The XAML document is a resource dictionary 
  which is merged into main client app resources, because it contains styles for our control.<br>
  <strong>It's even more weird because I created a class within main app that has attached property, then set property value in XAML markup - app compiled OK</strong></p>