title: How do I trigger a WPF expander IsExpanded property from another expander's IsExpanded property
body: <p>I have two expanders, side by side.  I want only one to be expanded at a time.  So if one is expanded, and the user expands the other, I want the first one to collapse.  The user can have both collapsed, and both collapsed is the starting state.</p>
  
  <p>As can be seen in the code, I have included the "Header" property as a test, and it works as expected, but the IsExpanded property is not working.</p>
  
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  <pre><code>&lt;Expander x:Name="emailExpander"&gt;
    &lt;Expander.Style&gt;
      &lt;Style TargetType="Expander"&gt;
        &lt;Setter Property="IsExpanded" Value="False"/&gt;
        &lt;Setter Property="Header" Value="Email"/&gt;
        &lt;Style.Triggers&gt;
          &lt;DataTrigger Binding="{Binding IsExpanded,ElementName=customerExpander}" Value="True"&gt;
            &lt;Setter Property="IsExpanded" Value="False"/&gt;
            &lt;Setter Property="Header" Value="other expanded"/&gt;
          &lt;/DataTrigger&gt;
        &lt;/Style.Triggers&gt;
      &lt;/Style&gt;
    &lt;/Expander.Style&gt;      
  &lt;/Expander&gt;
  </code></pre>