我做了一个三级联动的下拉列表框，后台用webservice,前台用webservice behavior与后台通讯。请高手们多提改进意见。

server端：（service1.asmx.cs)
using System;
using System.Text;
using System.Configuration;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Web;
using System.Web.Services;

namespace WebService1
{
    /// <summary>
    /// Summary description for Service1.
    /// </summary>
    public class Service1 : System.Web.Services.WebService
    {
        SqlConnection con;
        

        public Service1()
        {
            //CODEGEN: This call is required by the ASP.NET Web Services Designer
            InitializeComponent();
        }

        #region Component Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            string dsn = ConfigurationSettings.AppSettings["yitong"];

            con=new SqlConnection(dsn);
        }
        #endregion

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        protected override void Dispose( bool disposing )
        {
        }


        [WebMethod]
        public string fenlei()
        {
            string str="select distinct substring(code,1,3) as fenlei from jinan ";
            SqlCommand cmd=new SqlCommand(str,con);            

            cmd.Connection.Open();
            SqlDataReader dr=cmd.ExecuteReader();
            

            string s="请选择：";
            while(dr.Read())
            {
                s += ","+dr["fenlei"].ToString();