title: Animate Margin / Thickness
body: <p>I just thought I already know how WPF and XAML Syntax works.... 
  wrooong.</p>
  
  <p>I got the message:</p>
  
  <pre><code>'WithEvents' variables can only be typed as classes, interfaces or type parameters with class constraints.
  </code></pre>
  
  <p>Please, do you know why this syntax is wrong?</p>
  
  <p>I need to use the single value as double. It works later with an storyboard in codebehind.</p>
  
  <p>Regards
  <img src="https://i.stack.imgur.com/TONRC.png" alt="Error Screenshot"></p>
  
  <p>I just want to animate the Red rectangle with a storyboard in location and size. Perhaps XAML is the right solution anyway?
  <img src="https://i.stack.imgur.com/Gkegv.png" alt="Red rectangle"></p>