title: Add/Subtract GridLength in WPF
body: <p>I have three columns containing two expanders in left and right columns. When both the expnders collapsed, I need to arrage the content in the middle column on the full window. For that I need to calculate the gridlengths.</p>
  
  <p>For Example</p>
  
  <pre><code>GridLength w1= new GridLength( 20 );
  GridLength w2= new GridLength( 50 );
  GridLength w3= new GridLength( 0 );
  </code></pre>
  
  <p>How to get</p>
  
  <pre><code>w3 = w2 - w1
  </code></pre>