WPF中利用TreeView显示XML

<Window x:Class="WpfApplication1.MyFolderBrowserWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="浏览服务器文件夹" Height="300" Width="300"
        ResizeMode="NoResize" WindowStartupLocation="CenterOwner">
    <Window.Resources>       
        <HierarchicalDataTemplate  x:Key="ItemTemplate" ItemsSource="{Binding XPath=Folder}">
            <TextBlock Text="{Binding XPath=@Name}"/>
        </HierarchicalDataTemplate>
    </Window.Resources>
    <Grid>        
        <TreeView Name="TreeView"   ItemsSource="{Binding}" TreeViewItem.Expanded="TreeView_Expanded">
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate  ItemsSource="{Binding XPath=Folder}">
                    <StackPanel Orientation="Horizontal" Margin="2">
                        <Image Source="folder2.png" Width="16" Height="16" SnapsToDevicePixels="True"/>
                        <TextBlock Text="{Binding XPath=@Name}" Margin="5,0"/>
                    </StackPanel>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>
    </Grid>
</Window>

public partial class MyFolderBrowserWindow : Window
    {
        //字符串格式：<Folder Name=\"ServerName\"><Folder Name=\"C:\" FullPath=\"C:\\\"></Folder></Folder>
        public string RootFolders { get; set; }

        
        public MyFolderBrowserWindow()
        {
            InitializeComponent();
            this.Loaded += Window2_Loaded;
        }

        void Window2_Loaded(object sender, RoutedEventArgs e)
        {            
            XmlDataProvider xdp = new XmlDataProvider();
            xdp.XPath = "Folder";
            XmlDocument xd = new XmlDocument();
            xd.LoadXml(RootFolders);
            xdp.Document = xd;
            this.TreeView.Items.Clear();//Items和ItemSource，只能有一个生效，想用其中一个，另一个必须是空, but why items is not empty?
            this.TreeView.DataContext = xdp;
        }

        private void TreeView_Expanded(object sender, RoutedEventArgs e)
        {
            TreeViewItem item = e.OriginalSource as TreeViewItem;                    
            if(item != null)
            {
                if(item.HasItems)
                {
                    foreach (var childitem in item.Items)
                    {
                        XmlElement element = childitem as XmlElement;
                        if(!element.HasChildNodes)
                        {
                            string fullPath = element.GetAttribute("FullPath");
                            element.InnerXml = GetChildFolders(fullPath);//"<Folder Name=\"WPF\"/><Folder Name=\"MFC\"/><Folder Name=\"Delphi\"/>";
                        }                        
                    }
                }              
            }
        }
        public event Func<string, string> GetChildFoldersEvent;
        public delegate string GetChildFoldersEventHandler(object sender, FolderEventArgs args);
        public event GetChildFoldersEventHandler GetChildFoldersEvent2;
        private string GetChildFolders(string fullPath)
        {
            FolderEventArgs args = new FolderEventArgs();
            args.FullPath = fullPath;
            return GetChildFoldersEvent2(this, args);
            //return GetChildFoldersEvent(fullPath);
        }
    }

    public class FolderEventArgs : EventArgs
    {
        public string FullPath { get; set; }
    }