title: Override DependencyProperty in a UserControl
body: <p>I have a UserControl and the default value for <code>HorizontalContentAlignment</code> is <code>HorizontalAlignment.Stretch</code>. In the constructor I set it to <code>HorizontalAlignment.Left</code>.</p>
  
  <p>When I use the UserControl and give it the property <code>HorizontalAlignment.Right</code> in xaml, then that value is used, i.e. I cannot override the value in the constructor.</p>
  
  <p>I could override the property in <code>OnApplyTemplate</code>, <code>OnRender</code> or the <code>Loaded</code> event.  </p>
  
  <p>Is there any of these I should prefer?<br>
  Basically I want to avoid that someone can change the usercontrols <code>HorizontalContentAlignment</code>.</p>