title: Binding IsEnabled to Text.Length only works one way
body: <p>I have a TextBox, which I want to be enabled only if another TextBox has text in it. I am binding the Text.Length property of the first TextBox to the IsEnabled property on the second box. I have also tried binding the Text property of the first box and using a converter to convert to a bool. Both methods result in the second box being enabled when text is entered into the first but when the text is deleted the second box isn't disabled.</p>
  
  <p>I have tried setting NotifyOnSourceUpdated and NotifyOnTargetUpdated to true but neither has any effect.</p>
  
  <pre><code>&lt;TextBox Name="textBox1"/&gt;
  &lt;TextBox Name="textBox2" IsEnabled="{Binding ElementName=textBox1, Path=Text.Length}"/&gt;
  </code></pre>
  
  <p>So my question is what is needed for textBox2 to be disabled when the text in textBox1 is deleted.</p>