title: Multiple Binding targets if one value is null
body: <p>I'd like to bind a <code>Textblock.Text</code> to a class property which I've done, but when this value is null, I'd like a new property to be used and if that's null, then a default value to be set. I thought this is what PriorityBinding is for?</p>
  
  <pre><code>&lt;TextBlock.Text&gt;
      &lt;PriorityBinding&gt;
              &lt;Binding Path="Property1"/&gt;
              &lt;Binding Path="Property2"/&gt;
              &lt;Binding Source="---"/&gt;
      &lt;/PriorityBinding&gt;
  &lt;/TextBlock.Text&gt;
  </code></pre>