SelectedItem 获取 TreeView 中的选定项。
SelectedValue 获取 SelectedValuePath 属性指定的 SelectedItem 属性的值。

注意：（1）在生成节点之前，绑定事件（2）在生成完所有节点后，才设置默认节点

代码如下：

public FrmContactType()
        {
            InitializeComponent();
            trvMenu.SelectedItemChanged += new RoutedPropertyChangedEventHandler<object>(trvMenu_SelectedItemChanged);
            BindData();
        }

        void trvMenu_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
        {
            TreeViewItem item = trvMenu.SelectedItem as TreeViewItem;
        }

        private void BindData()
        {
            ParamWrapper param = new ParamWrapper();
            param.EntityType = typeof(TXType).AssemblyQualifiedName;

            EntityPortal.DAL.FetchSelf(param, (a, b) =>
            {
                _lstData = ListEx.ToList<object, TXType>(b.Result.ListData).ToList();

                if (trvMenu.Items.Count > 0)
                {
                    trvMenu.Items.Clear();
                }
                BuildTree(null, string.Empty);
                TreeViewItem item = trvMenu.Items.FirstOrDefault() as TreeViewItem;
                if (item != null)
                {
                    item.IsSelected = true;
                }               

               _lstData = null;
            });
        }