用ajax实现了联动，可是在后台代码中不能用dropdownlist2.SelectedItem.Text取得第二个的值，总是得到空值

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html

<%@ Page EnableEventValidation="false" Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
    <title>AjaxPro实现无刷新三级联动</title>
</head>
<script language="javascript" type="text/javascript">
      
</script>
<body>
    <form id="form1" runat="server">
    <div>
      <table width="500" border="0" align="center" cellpadding="0" cellspacing="0">
        <tr>
          <td width="99"> </td>
          <td width="401">
              城市<asp:DropDownList ID="ddl1" runat="server">
              </asp:DropDownList>
              区域<asp:DropDownList ID="ddl2" runat="server">
              </asp:DropDownList>
              花园<asp:DropDownList ID="ddl3" runat="server">
              </asp:DropDownList></td>
        </tr>
        <tr>
          <td> </td>
          <td> </td>
        </tr>
        <tr>
          <td> </td>
          <td> </td>
        </tr>
      </table>

    </div>
    </form>

</body>
</html>

using System;
using System.Data;
using System.Configuration;
using System.Data.OleDb;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        AjaxPro.Utility.RegisterTypeForAjax(typeof(_Default)); //必要的
        if(!IsPostBack)BindDc();
    }

    /// <summary>
    /// 数据库连接
    /// </summary>
    /// <returns></returns>
    public OleDbConnection myConn()
    {
        string ConnStr = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + System.Web.HttpContext.Current.Server.MapPath(ConfigurationManager.AppSettings["dbpath"]);
        OleDbConnection conn = new OleDbConnection(ConnStr);
        try
        {
            conn.Open();
            return conn;
        }
        catch
        {
            throw;
        }
    }

    /// <summary>
    /// 获取下级分类
    /// </summary>
    [AjaxPro.AjaxMethod]
    public DataSet getNextClass(string cid)
    {
        //因为不希望页面可以知道字段名称所以 as txt,id as vol 如果是sql ser 可以用 =
        //页面获取的列名称必须跟这个一同样而且区分大小写一般都是这个地方容易疏忽了
        //所以二级分类没变化
        string sql = @"select cname as txt,id as vol from webclass where parentid=" + cid;
        try
        {
            return getDs(sql);
        }
        catch
        {
            //throw;
            return null;
        }
    }

    /// <summary>
    /// 返回一个DataSet
    /// </summary>
    /// <param name="SQL"></param>
    /// <returns></returns>
    public DataSet getDs(string SQL)
    {
        OleDbConnection conn = myConn();
        DataSet Ds = new DataSet();
        OleDbDataAdapter Da = new OleDbDataAdapter(SQL, conn);
        try
        {
            Da.Fill(Ds);
            return Ds;
        }
        catch
        {
            return null;
            //throw;
        }

    }
    /// <summary>
    /// //数据绑定
    /// </summary>
    private void BindDc()
    {
        //第一个
        string sql = @"select * from webclass where Parentid=0";
        ddl1.DataSource = getDs(sql);
        ddl1.DataTextField = "cname";
        ddl1.DataValueField = "id";
        ddl1.DataBind();
        if (ddl1.DataSource != null) ddl1.Attributes.Add("onchange", "showNext(this.options[selectedIndex].value,'ddl2');");

        //可以先判断 DropDownList.SelectedItem.Value
        //第二个
        sql = @"select * from webclass where parentid=" + ddl1.SelectedItem.Value;
        ddl2.DataSource = getDs(sql);
        ddl2.DataTextField = "cname";
        ddl2.DataValueField = "id";
        ddl2.DataBind();

        //第三个
        if (ddl2.DataSource != null) ddl2.Attributes.Add("onchange", "showNext(this.options[selectedIndex].value,'ddl3');");
        sql = @"select * from webclass where parentid=" + ddl2.SelectedItem.Value;
        ddl3.DataSource = getDs(sql);
        ddl3.DataTextField = "cname";
        ddl3.DataValueField = "id";
        ddl3.DataBind();

    }
}

用ajax实现了联动，可是在后台代码中不能用dropdownlist2.SelectedItem.Text取得第二个的值，总是得到空值

用Request.Params[XXX.UniqueID] 即可获得该控件的值