title: How to save the IsExpanded state in group headers of a listview - Take 2
body: <p>To get some context this question refers to <a href="https://stackoverflow.com/questions/2808777/how-to-save-the-isexpanded-state-in-group-headers-of-a-listview/12343338#12343338">How to save the IsExpanded state in group headers of a listview</a></p>
  
  <p>What am I missing here?</p>
  
  <p>When I try to do what is suggested in the referenced post I get: </p>
  
  <p><strong>BindingExpression path error: '[]' property not found on 'object' ''ExpandStateManager'</strong></p>
  
  <p><strong>My ExpandStateManager</strong></p>
  
  <pre><code>public class ExpandStateManager
  {
      Dictionary&lt;Category, bool&gt; expandStates = new Dictionary&lt;Category, bool&gt;();
  
      public bool this[Category key]
      {
          get
          {
              if (!expandStates.ContainsKey(key)) return false;
              return expandStates[key];
          }
          set
          {
              expandStates[key] = value;
          }
      }
  }
  </code></pre>
  
  <p><strong>XAML</strong></p>
  
  <pre><code>    &lt;ListView ItemsSource="{Binding Source={StaticResource cvs}}"  ItemTemplate="{StaticResource animalTemplate}"
       Width="200"&gt;
          &lt;ListView.GroupStyle&gt;
              &lt;GroupStyle&gt;
                  &lt;GroupStyle.ContainerStyle&gt;
                      &lt;Style TargetType="{x:Type GroupItem}"&gt;
                          &lt;Setter Property="Template"&gt;
                              &lt;Setter.Value&gt;
                                  &lt;ControlTemplate TargetType="{x:Type GroupItem}"&gt;
                                      &lt;Expander IsExpanded="{Binding Source={StaticResource ExpandStateManger}, Path=[Items[0].ObjectType]}"&gt;
                                          &lt;Expander.Header&gt;
                                              &lt;DockPanel&gt;
                                                  &lt;TextBlock Text="{Binding Name}" /&gt;
                                              &lt;/DockPanel&gt;
                                          &lt;/Expander.Header&gt;
                                          &lt;Expander.Content&gt;
                                              &lt;ItemsPresenter /&gt;
                                          &lt;/Expander.Content&gt;
                                      &lt;/Expander&gt;
                                  &lt;/ControlTemplate&gt;
                              &lt;/Setter.Value&gt;
                          &lt;/Setter&gt;
                      &lt;/Style&gt;
                  &lt;/GroupStyle.ContainerStyle&gt;
              &lt;/GroupStyle&gt;
          &lt;/ListView.GroupStyle&gt;
      &lt;/ListView&gt;
  </code></pre>