title: Adding a user control "The Enumerator is not valid"
body: <p>I'm using Visual Studio 2010 to create a small WPF application.  I've created a user control that I am now trying to add to my main form.  The user control does show up in toolbox but every time I try to drag the control to the form I get the error:</p>
  
  <p><strong><em>The enumerator is not valid because the collection changed.</em></strong></p>
  
  <p>I should know what's wrong and it is bugging me that I can't figure it out.</p>