Introduction

This sample shows how to two-way-bind to the SelectedItem property of a WPF or Silverlight TreeView.

It also shows how to expand nodes down to the selected item, and collapse all other nodes (to keep it tidy).

Building the Sample

Just download, unzip, open and run!

You'll also need the Silverlight 4 SDK for the TreeView, which only comes from the SDK.

If you use a previous version of the SDK, just change the reference in the project.

Description

The problem with the TreeView is that the SelectedItem is a readonly property.

In silverlight, you don't even have OneWayToSource, so binding two way to SelectedItem (which does exist in XAML, even if intelisense doesn't show it) causes binding errors.

The solution is to add an Attached Property, which

1) Taps into the SelectedItemChanged event for target->source

2) Searches down the TreeViewItems for the object to select, for source->target.

In the process of searching down for the selected item, it has to expand the parent node to get it's children.

If it isn't in any of the descendants for a node, then the node gets closed again.

Behind all this is the following class:

using System.Windows;
using System.Windows.Controls;
 
namespace SilverlightTreeview
{
    public class Attached
    {
        public static object GetTreeViewSelectedItem(DependencyObject obj)
        {
            return (object)obj.GetValue(TreeViewSelectedItemProperty);
        }
 
        public static void SetTreeViewSelectedItem(DependencyObject obj, object value)
        {
            obj.SetValue(TreeViewSelectedItemProperty, value);
        }
 
        public static readonly DependencyProperty TreeViewSelectedItemProperty =
            DependencyProperty.RegisterAttached("TreeViewSelectedItem", typeof(object), typeof(Attached), new PropertyMetadata(new object(), TreeViewSelectedItemChanged));
 
        static void TreeViewSelectedItemChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            TreeView treeView = sender as TreeView;
            if (treeView == null)
            {
                return;
            }
 
            treeView.SelectedItemChanged -= new RoutedPropertyChangedEventHandler<object>(treeView_SelectedItemChanged);
            treeView.SelectedItemChanged += new RoutedPropertyChangedEventHandler<object>(treeView_SelectedItemChanged);
 
            TreeViewItem thisItem = treeView.ItemContainerGenerator.ContainerFromItem(e.NewValue) as TreeViewItem;
            if (thisItem != null)
            {
                thisItem.IsSelected = true;
                return;
            }
 
            for (int i = 0; i < treeView.Items.Count; i++)
                SelectItem(e.NewValue, treeView.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem);
 
        }
 
        static void treeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
        {
            TreeView treeView = sender as TreeView;
            SetTreeViewSelectedItem(treeView, e.NewValue);
        }
 
        private static bool SelectItem(object o, TreeViewItem parentItem)
        {
            if (parentItem == null)
                return false;
 
            bool isExpanded = parentItem.IsExpanded;
            if (!isExpanded)
            {
                parentItem.IsExpanded = true;
                parentItem.UpdateLayout();
            }
 
            TreeViewItem item = parentItem.ItemContainerGenerator.ContainerFromItem(o) as TreeViewItem;
            if (item != null)
            {
                item.IsSelected = true;
                return true;
            }
 
            bool wasFound = false;
            for (int i = 0; i < parentItem.Items.Count; i++)
            {
                TreeViewItem itm = parentItem.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem;
                var found = SelectItem(o, itm);
                if (!found)
                    itm.IsExpanded = false;
                else
                    wasFound = true;
            }
 
            return wasFound;
        }
    }
}
 
 
using System.Windows; 
using System.Windows.Controls; 
  
namespace SilverlightTreeview 
{ 
    public class Attached 
    { 
        public static object GetTreeViewSelectedItem(DependencyObject obj) 
        { 
            return (object)obj.GetValue(TreeViewSelectedItemProperty); 
        } 
  
        public static void SetTreeViewSelectedItem(DependencyObject obj, object value) 
        { 
            obj.SetValue(TreeViewSelectedItemProperty, value); 
        } 
  
        public static readonly DependencyProperty TreeViewSelectedItemProperty = 
            DependencyProperty.RegisterAttached("TreeViewSelectedItem", typeof(object), typeof(Attached), new PropertyMetadata(new object(), TreeViewSelectedItemChanged)); 
  
        static void TreeViewSelectedItemChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e) 
        { 
            TreeView treeView = sender as TreeView; 
            if (treeView == null) 
            { 
                return; 
            } 
  
            treeView.SelectedItemChanged -= new RoutedPropertyChangedEventHandler<object>(treeView_SelectedItemChanged); 
            treeView.SelectedItemChanged += new RoutedPropertyChangedEventHandler<object>(treeView_SelectedItemChanged); 
  
            TreeViewItem thisItem = treeView.ItemContainerGenerator.ContainerFromItem(e.NewValue) as TreeViewItem; 
            if (thisItem != null) 
            { 
                thisItem.IsSelected = true; 
                return; 
            } 
  
            for (int i = 0; i < treeView.Items.Count; i++) 
                SelectItem(e.NewValue, treeView.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem); 
  
        } 
  
        static void treeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e) 
        { 
            TreeView treeView = sender as TreeView; 
            SetTreeViewSelectedItem(treeView, e.NewValue); 
        } 
  
        private static bool SelectItem(object o, TreeViewItem parentItem) 
        { 
            if (parentItem == null) 
                return false; 
  
            bool isExpanded = parentItem.IsExpanded; 
            if (!isExpanded) 
            { 
                parentItem.IsExpanded = true; 
                parentItem.UpdateLayout(); 
            } 
  
            TreeViewItem item = parentItem.ItemContainerGenerator.ContainerFromItem(o) as TreeViewItem; 
            if (item != null) 
            { 
                item.IsSelected = true; 
                return true; 
            } 
  
            bool wasFound = false; 
            for (int i = 0; i < parentItem.Items.Count; i++) 
            { 
                TreeViewItem itm = parentItem.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem; 
                var found = SelectItem(o, itm); 
                if (!found) 
                    itm.IsExpanded = false; 
                else
                    wasFound = true; 
            } 
  
            return wasFound; 
        } 
    } 
} 
 
 
And below is an example of how to implement it:
 
<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" 
    x:Class="SilverlightApplication2.MainPage"
    xmlns:local="clr-namespace:SilverlightApplication2.Helpers"> 
  
    <StackPanel x:Name="LayoutRoot" > 
        <StackPanel.Resources> 
            <sdk:HierarchicalDataTemplate x:Key="ChildTemplate" ItemsSource="{Binding Path=Children}" > 
                <TextBlock Text="{Binding Path=Name}" /> 
            </sdk:HierarchicalDataTemplate> 
            <sdk:HierarchicalDataTemplate x:Key="NameTemplate" 
                ItemsSource="{Binding Path=Children}" 
                ItemTemplate="{StaticResource ChildTemplate}"> 
                <TextBlock Text="{Binding Path=Name}" FontWeight="Bold" /> 
            </sdk:HierarchicalDataTemplate> 
        </StackPanel.Resources> 
        <sdk:TreeView Width="400"  Height="300" 
            ItemsSource="{Binding HierarchicalAreas}" 
            ItemTemplate="{StaticResource NameTemplate}" x:Name="myTreeView"
            local:Attached.TreeViewSelectedItem="{Binding SelectedArea, Mode=TwoWay}" /> 
    </StackPanel> 
</UserControl>

 
This is a common request on MSDN WPF forum, so I hope you find this when you need it.