title: WPF Datatrigger not firing when expected
body: <p>I have the following XAML:    </p>
  
  <pre><code>&lt;TextBlock Text="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Margin="0,0,5,0"/&gt;
  &lt;TextBlock Text="items selected"&gt;
      &lt;TextBlock.Style&gt;
          &lt;Style TargetType="{x:Type TextBlock}"&gt;
              &lt;Style.Triggers&gt;
                  &lt;DataTrigger Binding="{Binding ElementName=EditListBox, Path=SelectedItems.Count}" Value="1"&gt;
                      &lt;Setter Property="TextBlock.Text" Value="item selected"&gt;&lt;/Setter&gt;
                  &lt;/DataTrigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
      &lt;/TextBlock.Style&gt;
  &lt;/TextBlock&gt;
  </code></pre>
  
  <p>The first text block happily changes with SelectedItems.Count, showing 0,1,2, etc.  The datatrigger on the second block never seems to fire to change the text.</p>
  
  <p>Any thoughts?</p>