title: DataTrigger to make WPF Button inactive until TextBox has value
body: <p>I want the <code>Button</code> control's property to be <code>IsEnabled="False"</code> until a value is entered into a <code>TextBox</code> in the <code>Window</code>.</p>
  
  <p><strong>Code so far:</strong></p>
  
  <pre><code>&lt;Button
    Content="Click Me"
    Name="ClickMe"
    VerticalAlignment="Top"
    Click="ClickMe_Click"&gt;
    &lt;Button.Style&gt;
      &lt;Style&gt;
        &lt;Style.Triggers&gt;
          &lt;DataTrigger
            Binding="{Binding ElementName=textBox, Path=Length}"
            &lt;!-- or even: Binding="{Binding Path=textBox.Length}" --&gt;
            Value="0"&gt;
            &lt;Setter
              Property="Button.IsEnabled"
              Value="false" /&gt;
            &lt;/DataTrigger&gt;
        &lt;/Style.Triggers&gt;
      &lt;/Style&gt;
    &lt;/Button.Style&gt;
  &lt;/Button&gt;
  </code></pre>
  
  <p>Also, is it possible to have this <code>Button</code> control's <code>IsEnabled</code> property be based on 3 different <code>TextBox</code> controls all having values?</p>