TREEVIEW绑定之后不能显示
以下是代码,为什么可以运行,但是在网页之中显示不出来  
一个显示页面Default.aspx  
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>  
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">  
<html xmlns="http://www.w3.org/1999/xhtml" >  
<head id="Head1" runat="server">  
     <title>无标题页 </title>  
</head>  
<body>  
     <form id="form1" runat="server">  
     <div>          
         <asp:TreeView ID="tvMenu" runat="server" ImageSet="Faq" ShowLines="True" ExpandDepth="0" Target="middle" >  
             <ParentNodeStyle Font-Bold="False" />  
             <HoverNodeStyle Font-Underline="True" ForeColor="Purple" />  
             <SelectedNodeStyle Font-Underline="True" HorizontalPadding="0px" VerticalPadding="0px" />  
             <NodeStyle Font-Names="Tahoma" Font-Size="8pt" ForeColor="DarkBlue" HorizontalPadding="5px"  
                 NodeSpacing="0px" VerticalPadding="0px" />  
         </asp:TreeView>      
     </div>  
     </form>  
</body>  
</html>  
一个代码隐藏页Default.aspx.cs  
using System;  
using System.Data;  
using System.Configuration;  
using System.Web;  
using System.Web.Security;  
using System.Web.UI;  
using System.Web.UI.WebControls;  
using System.Web.UI.WebControls.WebParts;  
using System.Web.UI.HtmlControls;  
using System.Data.SqlClient;  
public partial class _Default : System.Web.UI.Page  
{  
     SqlConnection Conn = new SqlConnection("server=20090207-0309;database=checkthree;uid=sa;pwd=1111");      
     DataSet ds;  
     protected void Page_Load(object sender, EventArgs e)  
     {  
         Conn.Open();  
         this.createDataSet();  
         Conn.Close();  
         this.InitTree(tvMenu.Nodes, "0");  
     }  
     private DataSet createDataSet()  
     {  
         ds = new DataSet();  
         string sqlStr = "select * from Tree ";  
         SqlDataAdapter cmdSelect = new SqlDataAdapter(sqlStr, Conn);  
         cmdSelect.Fill(ds, "Tree");  
         return ds;  
     }  
     protected void InitTree(TreeNodeCollection Nds, string parentId)//用递归方法动态生成节点  
     {  
         DataView dv = new DataView();  
         TreeNode tmpNode;  
         dv.Table = ds.Tables["Tree"];  
         dv.RowFilter = "pat_id=" + "'" + parentId + "'";  
         foreach (DataRowView drv in dv)  
         {  
             tmpNode = new TreeNode();  
             tmpNode.Value = drv["node_id"].ToString();  
             tmpNode.Text = drv["naode_name"].ToString();  
             tmpNode.NavigateUrl = drv["url"].ToString();  
             Nds.Add(tmpNode);