Read TreeView SelectedItem

TreeViewExtensions.cs

public static class TreeViewExtensions
{
    private static readonly object initialBindingTarget = new object();

    public static object GetSelectedItem(DependencyObject obj) => 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) =>
        {
            if (!(d is TreeView treeView))
                return;

            // Trick to see if this is the first time we're set
            if (e.OldValue == initialBindingTarget)
            {
                treeView.SelectedItemChanged += SelectedItemChanged;
            }
        }));

    private static void SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        if (!(sender is TreeView treeView))
            return;

        SetSelectedItem(treeView, e.NewValue);
    }
}