ASP中TreeView用递归填充数据

using System;
using System.Data;
using System.Configuration;
using System.Collections;
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 TreeView_Default : System.Web.UI.Page
{
    string stringConn = "server=.;uid=sa;pwd=sa;database=zf";
    protected void Page_Load(object sender, EventArgs e)
    {
        addNode();
    }
    private void addNode() //添加根节点
    {
        SqlConnection conn = new SqlConnection(stringConn);
        SqlCommand cmd = new SqlCommand("select * from City where TID=0",conn);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dtSource = new DataTable("parentNode");
        da.Fill(dtSource);

        foreach (DataRow row in dtSource.Rows) 
        {
            TreeNode node = new TreeNode(row["title"].ToString(),row["CID"].ToString());
            

            addChildNode(node);
            this.treeview1.nodes.add(node);
        }
    }
    private void addChildNode(TreeNode childNode) //添加所有子节点
    {
        SqlDataAdapter da = new SqlDataAdapter(string.Format("select * from City where tid={0}", childNode.Value), stringConn);
        DataTable dtSource = new DataTable("childNode");
        da.Fill(dtSource);

        foreach (DataRow row in dtSource.Rows)
        {
            TreeNode node = new TreeNode(row["title"].ToString(), row["CID"].ToString());
            addChildNode(node);
            childNode.ChildNodes.Add(node);
        }
    }