ASP.net DropDownList 无刷新联动菜单

<script language="javascript">    function load(state)    {     var drp2 = document.getElementById("DropDownList2");     for(var i=0;i<=drp2.options.length-1;i++)     {      drp2.remove(i);     }     var oHttpReq = new ActiveXObject("MSXML2.XMLHTTP");    //发送请求     var oDoc = new ActiveXObject("MSXML2.DOMDocument");    //响应结果     var state=document.getElementById("DropDownList1").value;     oHttpReq.open("post","webform2.aspx?state="+state, false);     oHttpReq.send("");     result = oHttpReq.responseText;     oDoc.loadXML(result);　         // items = oDoc.selectNodes("//CITY/Table");        items = oDoc.selectNodes("//address/Table"); 　　   for (var item = items.nextNode();item;item = items.nextNode()) 　　   {　 　　   　var city = item.selectSingleNode("//address").nodeTypedValue; 　　   　var newOption = document.createElement("OPTION");　 　　   　newOption.text = city;　 　　   　newOption.value = city;　 　　   　drp2.options.add(newOption);　 　　  　 }　    }   </script>

============================

webform1.aspx

private void Page_Load(object sender, System.EventArgs e)  {   // 在此处放置用户代码以初始化页面   if(!IsPostBack)   {    SqlConnection con = new SqlConnection("packet size=4096;user id=sa;data source=SERVER;persist security info=False;initial catalog=pubs");    SqlDataAdapter da = new SqlDataAdapter("select state from authors group by state",con);    DataSet ds = new DataSet("State");    da.Fill(ds);    this.DropDownList1.DataSource = ds.Tables[0];    this.DropDownList1.DataTextField = "state";    this.DropDownList1.DataValueField = "state";　    this.DropDownList1.DataBind();　　    this.DropDownList1.Attributes.Add("onchange","load()");　　    //DropDownList1.   }    }

webform2.aspx

 private void Page_Load(object sender, System.EventArgs e)  {   // 在此处放置用户代码以初始化页面   if(this.Request["state"]!=null)   {　　    string state = this.Request["state"].ToString();　　    SqlConnection con = new SqlConnection("packet size=4096;user id=sa;data source=SERVER;persist security info=False;initial catalog=pubs");    SqlDataAdapter da = new SqlDataAdapter("select address from authors where state = '"+state+"'",con);　　    DataSet ds = new DataSet("address");　　    da.Fill(ds);　　    XmlTextWriter writer = new XmlTextWriter(Response.OutputStream, Response.ContentEncoding);　　    writer.Formatting = Formatting.Indented;　　    writer.Indentation = 4;　　    writer.IndentChar = ' ';　　    ds.WriteXml(writer);　　    writer.Flush();　　    Response.End();　　    writer.Close();　　   }  }