title: Changing Content of Window from Button Click in User Control
body: <p>I am working on a program that has a main window with only a <code>ContentControl</code> element.
   In the class for this main window, I can change the content by <code>ContentHolder.Content = new UserControlMain()</code>. In <code>UserControlMain</code>, I have a button to once again change the content of the window to a new user control.</p>
  
  <p>I am running into issues with changing the window content the second time. If I do not instantiate a class of <code>MainWindow</code>, I cannot access the <code>Content</code> property from anywhere except its own class. I also cannot access the <code>ContentControl</code> element I have associated with that class. However, if I do instantiate a <code>MainWindow</code> object, I run into an error message stating that the content has already been set.</p>
  
  <p>Any tips on how I may get around this?</p>