title: WPF HierarchicalDataTemplate doesn't update ItemsSource on property changed
body: <p>Here is some XAML</p>
  
  <pre><code>&lt;HierarchicalDataTemplate DataType="{x:Type data:FolderEntity}" 
    ItemsSource="{Binding Path=FolderEntities,UpdateSourceTrigger=PropertyChanged}"&gt;
    &lt;Label Content="{Binding FolderName}"/&gt;
  &lt;/HierarchicalDataTemplate&gt;
  &lt;TreeView/&gt;
  </code></pre>
  
  <p>data:FolderEntity is a LINQ to SQL data class which implements the INotifyPropertyChanging and INotifyPropertyChanged interfaces.</p>
  
  <p>My problem is that when I change the FolderEntities property the binding doesn't update. If I change the FolderName property the tree node that corresponds to that item will change, but the collection that is FolderEntities just wont.</p>
  
  <p>I'm thinking WPF checks to see whether the collection reference has changed, or does the ItemsSource object have to be an ObservableCollection`1 for this to work?</p>
  
  <p>Any input on the matter is much appreciated.</p>