TreeViewExtensions.cs

/// <summary>
/// Extensions on TreeViews allowing users to bind to the SelectedItem property
/// </summary>
/// <remarks>
/// The trick used to detect when the binding has occurred means that there are some cases it doesn't handle - but I don't
/// recall what.
/// </remarks>
public static class TreeViewExtensions
{
    private static readonly object initialBindingTarget = new object();

    public static object GetSelectedItem(DependencyObject obj)
    {
        return obj.GetValue(SelectedItemProperty);
    }

    public static void SetSelectedItem(DependencyObject obj, object value)
    {
        obj.SetValue(SelectedItemProperty, value);
    }

    public static readonly DependencyProperty SelectedItemProperty =
        DependencyProperty.RegisterAttached("SelectedItem", typeof(object), typeof(TreeViewExtensions), new FrameworkPropertyMetadata(initialBindingTarget, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, (d, e) =>
        {
            var treeView = d as TreeView;
            if (treeView == null)
                return;

            // Trick to see if this is the first time we're set
            if (e.OldValue == initialBindingTarget)
            {
                treeView.SelectedItemChanged += SelectedItemChanged;
                treeView.ItemContainerGenerator.StatusChanged += (o2, e2) => ItemContainerGeneratorStatusChanged(treeView);
            }

            var container = ContainerFromItem(treeView.ItemContainerGenerator, e.NewValue);
            if (container != null)
            {
                container.IsSelected = true;
            }
        }));

    private static void ItemContainerGeneratorStatusChanged(TreeView treeView)
    {
        var containerGenerator = treeView.ItemContainerGenerator;
        if (containerGenerator.Status != GeneratorStatus.ContainersGenerated)
            return;

        var container = ContainerFromItem(containerGenerator, GetSelectedItem(treeView));
        if (container != null)
        {
            container.IsSelected = true;
        }
    }

    private static TreeViewItem ContainerFromItem(ItemContainerGenerator containerGenerator, object item)
    {
        // From http://stackoverflow.com/a/24859863/1086121

        var container = containerGenerator.ContainerFromItem(item) as TreeViewItem;
        if (container != null)
        {
            return container;
        }

        foreach (object childItem in containerGenerator.Items)
        {
            var parent = containerGenerator.ContainerFromItem(childItem) as TreeViewItem;
            if (parent == null)
                continue;

            container = parent.ItemContainerGenerator.ContainerFromItem(item) as TreeViewItem;
            if (container != null)
                return container;

            container = ContainerFromItem(parent.ItemContainerGenerator, item);
            if (container != null)
                return container;
        }

        return null;
    }

    private static void SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        var treeView = sender as TreeView;
        if (sender == null)
            return;

        SetSelectedItem(treeView, e.NewValue);
    }
}