动态更新下拉列表

 ----------------前台aspx----------------------------
       function getCity(str)
        {      
         WebService.GetCity(str,OnSuccess);
         }
         function OnSuccess(ary)
         {
        var list=document.getElementById("route");
        list.options.length=0;
        for(var i=0,j=ary.length;i<j;i++)
        {
        list.options[i]=new Option(ary[i].P,ary[i].I);
        }
         }
----------------------------------------------
后台aspx.cs
------------------------------------------------
using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Data;
using System.Data.SqlClient;
using System.Collections.Generic;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack )
        diquclass.Attributes["onchange"] = "getCity(this.value)";
    }
-------------------------------------------------
WebService.cs
*-------------------------------------------

using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Data;
using System.Data.SqlClient;
using System.Collections.Generic;

    /// <summary>
    /// WebService 的摘要说明
    /// </summary>
    [Serializable]
    public class Province
    {
        public string P;
        public int I;
        public Province() { }
        public Province(string p, int i)
        {
            P = p;
            I = i;
        }
    }
    [System.Web.Script.Services.ScriptService]
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    public class WebService : System.Web.Services.WebService
    {

        public WebService()
        {

            //如果使用设计的组件，请取消注释以下行
            //InitializeComponent();
        }
        //自己可以产生一个Json字符串呀.

        [System.Web.Services.WebMethod]
        [System.Web.Script.Services.ScriptMethod]
        public Province[] GetCity(int pid)
        {
            List<Province> ary = new List<Province>();
            DataTable dt = framework.route(pid.ToString ());
            for (int i = 0; i <= dt.Rows.Count-1; i++)
            {
                ary.Add(new Province(dt.Rows[i]["route"].ToString(), Convert.ToInt32(dt.Rows[i]["id"])));
            }
            dt.Dispose();
            return ary.ToArray();

        }

    }