PopulatingEventArgs Class

public partial class MainPage : UserControl
{
    // Create the topics collection.
    static public ObservableCollection<Topic> Topics =
        new ObservableCollection<Topic>();
    public MainPage()
    {
        InitializeComponent();

        // Add some topics to the collection.
        Topic UsingControls = new Topic("Using Controls and Dialog Boxes", Guid.NewGuid());
        UsingControls.ChildTopics.Add(new Topic("Getting Started with Controls", Guid.NewGuid()));
        Topic DataGridTopic = new Topic("DataGrid", Guid.NewGuid());
        DataGridTopic.ChildTopics.Add(
            new Topic("Default Keyboard and Mouse Behavior in the DataGrid Control",
                Guid.NewGuid()));
        DataGridTopic.ChildTopics.Add(
            new Topic("How to: Add a DataGrid Control to a Page",
                Guid.NewGuid()));
        DataGridTopic.ChildTopics.Add(
            new Topic("How to: Display and Configure Row Details in the DataGrid Control",
                Guid.NewGuid()));
        UsingControls.ChildTopics.Add(DataGridTopic);
        UsingControls.ChildTopics.Add(new Topic("Ink Presenter", Guid.NewGuid()));
        Topics.Add(UsingControls);
        myTreeView.DataContext = Topics;
    }

    void myTreeView_MouseEnter(object sender, MouseEventArgs e)
    {
        ExpandAll(sender, e);
    }

    // Method to expand all the treeview children.
    private void ExpandAll(object sender, RoutedEventArgs e)
    {
        for (int i = 0; i < myTreeView.Items.Count; i++)
        {
            TreeViewItem childItem =
                myTreeView.ItemContainerGenerator.ContainerFromItem(myTreeView.Items[i])
                    as TreeViewItem;
            if (childItem != null)
                ExpandAllTreeViewItems(childItem);
        }
    }

    // Declare a delegate for the method to expand all children. You'll do this
    // asynchronously to ensure the items are instatiated before they are expanded. 
    public delegate void ExpandChildren(TreeViewItem currentTreeViewItem);

    // Exapand all the children of the current treeview item.
    private void ExpandAllTreeViewItems(TreeViewItem currentTreeViewItem)
    {
        // If the current node is not expanded, call the expansion asynchronously 
        // to ensure the items are instantiated before expansion.
        if (!currentTreeViewItem.IsExpanded)
        {
            currentTreeViewItem.IsExpanded = true;

            // Asynchronous  call to expand the children.
            currentTreeViewItem.Dispatcher.BeginInvoke(new
                ExpandChildren(ExpandAllTreeViewItems),
                new object[] { currentTreeViewItem });

            // Alternatively, you could make the call to ExpandAllTreeViewItems
            // using lambda syntax.
            // currentTreeViewItem.Dispatcher.BeginInvoke(() => 
            //   ExpandAllTreeViewItems(currentTreeViewItem));
        }

        else
        {
            for (int i = 0; i < currentTreeViewItem.Items.Count; i++)
            {
                TreeViewItem child = (TreeViewItem)
                    currentTreeViewItem.ItemContainerGenerator.ContainerFromIndex(i);
                ExpandAllTreeViewItems(child);
            }
        }
    }

    // Simple business object.
    public class Topic
    {
        public string Title { get; set; }
        public Guid Id { get; set; }
        public ObservableCollection<Topic> ChildTopics { get; set; }
        public Topic()
        {
            ChildTopics = new ObservableCollection<Topic>();
        }
        public Topic(string title, Guid idValue)
            : this()
        {
            Title = title;
            Id = idValue;
        }
    }
}

<StackPanel x:Name="LayoutRoot" Background="White">
    <StackPanel.Resources>
        <sdk:HierarchicalDataTemplate x:Key="ChildTemplate" ItemsSource="{Binding Path=ChildTopics}"  >
            <TextBlock FontStyle="Italic" Text="{Binding Path=Title}"  />
        </sdk:HierarchicalDataTemplate>
        <sdk:HierarchicalDataTemplate x:Key="RootTemplate" 
            ItemsSource="{Binding Path=ChildTopics}" 
            ItemTemplate="{StaticResource ChildTemplate}">
            <TextBlock Text="{Binding Path=Title}" FontWeight="Bold" />
        </sdk:HierarchicalDataTemplate>
    </StackPanel.Resources>
    <sdk:TreeView Width="400" Height="200" ItemsSource="{Binding}" 
        ItemTemplate="{StaticResource RootTemplate}" x:Name="myTreeView" 
        MouseEnter="myTreeView_MouseEnter" />
  </StackPanel>