TreeView.SelectedValue Property

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.
        Topics.Add(new Topic("Using Controls and Dialog Boxes", Guid.NewGuid()));
        Topics.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()));
        Topics.Add(DataGridTopic);
        myTreeView.DataContext = Topics;

        //Associated a handler with the item changed event.
        myTreeView.SelectedItemChanged += 
            new RoutedPropertyChangedEventHandler<object>(myTreeView_SelectedItemChanged);
    }

    void myTreeView_SelectedItemChanged(object sender, 
        RoutedPropertyChangedEventArgs<object> e)
    {
        // Set the data context of the text block to the selected value.
        TreeView myTreeView = sender as TreeView;
        idTextBlock.DataContext = myTreeView.SelectedValue;
    }
}

// Simple business object.
public class Topic
{
    public string Title { get; set; }
    public Guid Id { get; set; }
    public ObservableCollection<Topic> ChildTopics { get; set; }
    private 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" >
                <TextBlock FontStyle="Italic" Text="{Binding Path=Title}" />
            </sdk:HierarchicalDataTemplate>
            <sdk:HierarchicalDataTemplate x:Key="NameTemplate" 
                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 NameTemplate}" x:Name="myTreeView" 
	    SelectedValuePath="Id" />
        <StackPanel Orientation="Horizontal" >
            <TextBlock Margin="5" Text="Selected topic GUID:" />
            <TextBlock FontWeight="Bold" Margin="5" Text="{Binding}" x:Name="idTextBlock" />
        </StackPanel>
            </StackPanel>