无刷新实现 DropDownList 联动

       前段时间，我在利用ajaxAjaxControlToolkit插件时，发现有时它的值不值为何无法取值。 后我写了一个利用AJAX技术写的一个DropDownList联动的；
      下面是它的内容：
        <form id="form1" runat="server">
        <asp:DropDownList ID="DropDownList1" runat="server" Style="position: relative">
        </asp:DropDownList>
        <asp:DropDownList ID="DropDownList2" runat="server" onchange= "Change()">
        </asp:DropDownList>
        <script type="text/javascript" language="javascript">
        <!--
            function XmlPost(Object)//联动取值绑定DropDownList2。后台实现无刷新绑定操作；
            {
               var svalue = Object.value;
               var webFileUrl = "?ZoneID=" + svalue;
               var result = "";
　　           var xmlHttp = new ActiveXObject("MSXML2.XMLHTTP");
　　           xmlHttp.open("POST", webFileUrl, false);
　　           xmlHttp.send("");
　　           result = xmlHttp.responseText;

                if(result != "" && result != "----" )
　　                {
　　　　                document.all("DropDownList2").length=0;
　　　　                var piArray = result.split(",");
　　　　                //分离字符串并绑定数据
　　　　                for(var i=0;i<piArray.length;i++)
　　　　                    {
　　　　　　                    var ary1 = piArray[i].toString().split("|");
　　　　　　                    document.all("DropDownList2").options.add(new Option(ary1[1].toString(),ary1[0].toString()));
　　　　　　                    document.getElementById("Label1").innerText = document.getElementById("DropDownList2").value;
　　　　                    }
　　                }
　　            else if( result == "----")
　　                {
　　                     document.all("DropDownList2").length=0;
　　                     document.all("DropDownList2").options.add(new Option("--不限--","--不限--"));
　　　　                 document.getElementById("Label1").innerText = "--不限--";
　　                }
　　            else
　　                {
　　　　                alert(result);
　　                }
            }
           
            function Change()
            {
                 document.getElementById("Label1").innerText = document.getElementById("DropDownList2").value;
            }
            
          -->
        </script>
        <asp:Label ID="Label1" runat="server" Style="position: relative" Text="City"></asp:Label>
    </form>

              后台代码：
                

    protected void Page_Load(object sender, EventArgs e)
    {
        string brc_id = this.Request.QueryString["ZoneID"];
        if (brc_id != null)
        {
            this.down2_bind(brc_id);
        }
        else
        {
            if (!this.IsPostBack)
            {
                this.down1_bind();
                this.DropDownList2.Items.Clear();
                this.DropDownList2.Items.Add("--不限--");
                this.DropDownList2.SelectedIndex = this.DropDownList2.Items.Count - 1;
            }
        }
    }

    private void down1_bind()// 绑定DropDownList1数据
    {
       
       
        DataTable mytab = bll.GetPrivnice(1);
        this.DropDownList1.DataSource = mytab;
        this.DropDownList1.DataValueField = "ZoneID";
        this.DropDownList1.DataTextField = "ZoneName";
        this.DropDownList1.DataBind();
        this.DropDownList1.Items.Add("--不限--");
        this.DropDownList1.SelectedIndex = this.DropDownList1.Items.Count - 1;
        this.DropDownList1.Attributes.Add("onchange", "XmlPost(this);");//当DropDownList变动时，联动DropDownList2的值。
    }

    private void down2_bind(string brc_id)// 取得/绑定DropDownList1数
    {

        int a = 1;
        string mystr = "";
        if (brc_id != "" && brc_id != "----")
        {//将取出的若干值串成字符串
            a = int.Parse(brc_id);
            DataTable mytab = bll.GetPrivnice(a);//返回table暂存于内存
            if (mytab.Rows.Count != 0)
            {
                for (int i = 0; i < mytab.Rows.Count; i++)
                {
                    mystr += "," + mytab.Rows[i][1].ToString() + "|" + mytab.Rows[i][2].ToString();
                }
                mystr = mystr.Substring(1);
            }
            this.Response.Write(mystr);
            this.Response.End();
        }
        else if (brc_id == "----")//如果为不限时DropDownList2绑定为不限
        {
            mystr = brc_id;
            this.DropDownList2.Items.Clear();
            this.DropDownList2.Items.Add("--不限--");
            this.DropDownList2.SelectedIndex = this.DropDownList2.Items.Count - 1;
            this.Response.Write(mystr);
            this.Response.End();
        }

        嘻嘻！初学AJAX技术。