title: How to make custom WPF ContentControl that supports drag-and-drop at design time?
body: <p>I want to create custom WPF control that has a single "child" control inside. Subclassing <code>ContentControl</code> or <code>UserControl</code> works, but has one flaw: these controls don't work in designer mode.</p>
  
  <p>By "don't work" I mean this scenario: suppose I have a <code>Canvas</code> with my custom control in it. I want to put, say, a <code>Button</code> inside my control. I drag it from the toolbox, and it <em>appears</em> inside my control. However, XAML view shows that the new button actually belongs to <code>Canvas</code>, not to my control. </p>
  
  <p>I can place it inside my control by manually editing XAML, but I want the designer to work too.</p>
  
  <p>Interestingly, when I subclass <code>Canvas</code>, <code>Grid</code> or <code>Panel</code>, designer works as expected. However, these controls have <em>many</em> children, which is not what I need. </p>
  
  <p>How can I make a single-child control that works in designer?</p>