dropdownlist绑定数据后，进行插入数据错误

现已将两个dropdownlist关联，选中第一个dropdownlist表示一个大类进行选择后，第二个dropdownlist会显示出第一个的相应子类。但是插入操作时，第二个dropdownlist中插入的数据永远都是子类中的第一个数据，我想了好久都没有明白，望高人指点。
以下是源码关键问题是：（第一个dropdownlist是ddlclass,第二个dropdownlist是ddldevice），ddlclass是设备的大类别，ddldevice是设备的编号，选中大类别后再去选择编号，但是插入的设备编号却永远是排在一个个的编号，而非选中的编号
前台：
后台代码
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Xml.Linq;
using System.Data.SqlClient;
namespace ServiceDesk.SD
{
    public partial class sduseless : System.Web.UI.Page
    {
        // 定义连接字符串
        private static string strCon = System.Configuration.ConfigurationManager.AppSettings["connStr"];
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                //绑定ddlclass 
                SqlConnection con = new SqlConnection(strCon);
                con.Open();
                SqlCommand cmd = new SqlCommand("select * from class ", con);
                SqlDataReader reader = cmd.ExecuteReader();
                ddlclass.DataSource = reader;
                ddlclass.DataTextField = "classname";
                ddlclass.DataValueField = "classno";
                ddlclass.DataBind();
                reader.Close();                //绑定ddldevice 
                SqlCommand cmd2 = new SqlCommand("select * from deviceinfo where classno=" + this.ddlclass.SelectedValue, con);
                reader = cmd2.ExecuteReader();//上面定义过reader，这里就不用从新定义 
                ddldevice.DataSource = reader;
                ddldevice.DataTextField = "deviceno";
                ddldevice.DataValueField = "classno";
                ddldevice.DataBind();
                reader.Close();
            }
        }
        // 验证用户名
        private bool blnCheckUser()
        {
            // 数据库连接
            SqlConnection objConnection = new SqlConnection(strCon);
            // 数据库命令
            SqlCommand objCommand = new SqlCommand("", objConnection);
            // 设置Sql语句
            objCommand.CommandText =
                " SELECT userno FROM USERS WHERE userno = @user";
            // 设置Sql语句参数
            objCommand.Parameters.Add("user", SqlDbType.VarChar);
            // Sql语句参数赋值
            objCommand.Parameters["user"].Value = txtuserno.Text.Trim();            try
            {
                // 打开数据库连接
                if (objConnection.State == ConnectionState.Closed) objConnection.Open();
                // 获取运行结果
                SqlDataReader result = objCommand.ExecuteReader();
                if (result.Read() == false)
                {
                    lblCheck.Visible = true;
                    return false;
                }
                else
                {
                    lblCheck.Visible = false;
                    return true;
                }
            }
            catch (SqlException exp)
            {
                Session["Error"] = exp.Message;
                Response.Redirect("Error.aspx");                return false;
            }
            finally
            {
                // 关闭数据库连接
                if (objConnection.State == ConnectionState.Open) objConnection.Close();
            }
        }
        protected void ddlclass_SelectedIndexChanged(object sender, EventArgs e)
        {
            //ddlclass与ddldevice数据绑定的联动性 
            string code = this.ddlclass.SelectedValue;
            SqlConnection con = new SqlConnection(strCon);
            con.Open();
            SqlCommand cmd = new SqlCommand("select * from deviceinfo where classno=" + code, con);
            SqlDataReader reader = cmd.ExecuteReader();
            this.ddldevice.DataSource = reader;
            this.ddldevice.DataTextField = "deviceno";
            this.ddldevice.DataValueField = "classno";
            this.ddldevice.DataBind();
            reader.Close();
            con.Close();
        }
        protected void btnRegion_Click(object sender, EventArgs e)
        {
            if (blnCheckUser() == false) return;            // 数据库连接
            SqlConnection conn = new SqlConnection(strCon);
            // 数据库命令
            SqlCommand objCommand = new SqlCommand("", conn);
            // 设置Sql语句          
                objCommand.CommandText =
                "insert INTO uselesslist (userno, classno, deviceno)" +
                "values (@userno,@classno,@deviceno) "; 
            
            // 设置Sql语句参数
            objCommand.Parameters.Add("userno", SqlDbType.VarChar);
            objCommand.Parameters.Add("classno", SqlDbType.VarChar);
            objCommand.Parameters.Add("deviceno", SqlDbType.VarChar);
            
            // Sql语句参数赋值
            objCommand.Parameters["userno"].Value = txtuserno.Text.Trim();
            objCommand.Parameters["classno"].Value = ddlclass.Text.Trim();
            objCommand.Parameters["deviceno"].Value = ddldevice.SelectedItem.ToString();
            try
            {
                // 打开数据库连接
                if (conn.State == ConnectionState.Closed) conn.Open();
                // 插入数据
                objCommand.ExecuteNonQuery();
            }
            catch (SqlException exp)
            {
                Session["Error"] = exp.Message;
                Response.Redirect("Error.aspx");
            }
            finally
            {
                // 关闭数据库连接
                if (conn.State == ConnectionState.Open) conn.Close();
            }
            Response.Redirect("2.aspx");
        }
        protected void btnClear_Click(object sender, EventArgs e)
        {
            txtuserno.Text = "";
            
        }
    }
}