title: Binding to Style Target using RelativeSource in Setter.Value
body: <p>I want to create a Trigger to be applied to all TextBox on Validation.HasError to show the Validation.Error in a custom ToolTip.</p>
  
  <pre><code>&lt;Style TargetType="{x:Type TextBox}"&gt;
      &lt;Style.Triggers&gt;
          &lt;Trigger Property="Validation.HasError" Value="True"&gt;
          &lt;Setter Property="Background" Value="Red"/&gt;
          &lt;Setter Property="ToolTip"&gt;
              &lt;Setter.Value&gt;
                  &lt;StackPanel&gt;
                      &lt;TextBlock Text="{Binding RelativeSource={RelativeSource XXX}, Path=(Validation.Error)[0].ErrorContent}"/&gt;
                  &lt;/StackPanel&gt;
              &lt;/Setter.Value&gt;
          &lt;/Setter&gt;
          &lt;/Trigger&gt;
      &lt;/Style.Triggers&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p><strong>What should I put in the XXX?</strong></p>
  
  <p>My problem is that I don't really understand how RelativeSource works in this context and I can't get the correct code for binding to the TextBox. </p>
  
  <p>I am guessing Self would refer to the TextBlock and FindAncestor x:Type TextBox will fail becuase it will traverse from TextBlock > StackPanel > Setter.Value > Setter > etc.. instead.</p>
  
  <p><strong>How can I refer to the Style Target instead?</strong></p>