title: Custom Expression to Define Width and Height in WPF
body: <p>I'm new in WPF so this is a very dummy question.</p>
  
  <p>In Visual Studio -> Properties when I select a StackPanel (for example), I have the property Width. In this property I can click in the little square on the right and a menu is open. One of the option in the menu is "Custom Expression"</p>
  
  <p><strong>So here is my question:</strong><br>
  Is possible to define Width and Height base in a Mathmatic expression?</p>
  
  <pre><code>&lt;StackPanel Width="{Parent.Width - 100}"&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p>Or something like that?</p>
  
  <p><strong>EDIT</strong><br>
  I'm asking this because i'm intend to create a StackPanel that need to have a width 100 pixels lower than the Window. When window size was changed the StackPanel need to change to corresponding to this rule.</p>