title: Set to visible when item in a combobox is selected
body: <p>As the title says, I have a hidden border with some controls inside, and I would like to show it when a particular item in a combobox is selected. </p>
  
  <p>I tried the following </p>
  
  <pre><code>&lt;ComboBox Name="cmbRequiredRule" SelectedValuePath="Content"                                                                                                    
            SelectedValue="{Binding Path=ClientValidation.NarrativeRequiredRule}"&gt;
      &lt;ComboBoxItem&gt;All&lt;/ComboBoxItem&gt;                         
      &lt;ComboBoxItem&gt;Matching&lt;/ComboBoxItem&gt;
  &lt;/ComboBox&gt;
  
  &lt;Border Visibility="Collapsed"&gt;
      &lt;Border.Resources&gt;
          &lt;Style TargetType="{x:Type Border}"&gt;
              &lt;Style.Triggers&gt;
                  &lt;DataTrigger Binding="{Binding Path=ClientValidation.NarrativeRequiredRule}" Value="Matching"&gt;
                      &lt;Setter Property="Visibility" Value="Visible" /&gt;
                  &lt;/DataTrigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
      &lt;/Border.Resources&gt; 
  .... 
  &lt;/Border&gt;
  </code></pre>
  
  <p>and this property in the view model: </p>
  
  <pre><code> public string NarrativeRequiredRule
      {
          get... 
          set... 
      } 
  </code></pre>
  
  <p>but the trigger doesn't seem to be working</p>