阅读器关闭时尝试调用Read无效

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.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

namespace DepartMent
{
    public partial class _Default : System.Web.UI.Page
    {
        // protected System.Web.UI.WebControls.ListBox lBoxEmp;
        protected System.Web.UI.WebControls.DropDownList ddlEmp;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                SqlConnection con = DBCon.createConnection();
                con.Open();
                //显示部门
                SqlCommand cmd = new SqlCommand("select * from TDepartment", con);//定义连接                   SqlDataReader sdr = cmd.ExecuteReader();
                this.ddlDep.DataSource = sdr;
                this.ddlDep.DataTextField = "depName";
                this.ddlDep.DataValueField = "depID";
                this.ddlDep.DataBind();
                sdr.Close();
                
                //显示员工
                SqlCommand cmdEmp = new SqlCommand("select * from emp where depID=" + this.ddlDep.SelectedValue, con);
                SqlDataReader sdrEmp=cmdEmp.ExecuteReader();
                while (sdrEmp.Read())
                {
                    this.lBoxEmp.Items.Add(new ListItem(sdrEmp.GetString(1), sdrEmp.GetInt32(0).ToString()));
                    sdrEmp.Close();
                    //关闭连接
                    con.Close();

                }

            }

        }

就是SqlDataReader sdrEmp=cmdEmp.ExecuteReader();这行出错，阅读器关闭时尝试调用Read无效，堆栈
我添加了一个类,然后在WebForm1里拽了个DropDownList和一个ListBox,
在线等待ing...
--------------------编程问答-------------------- 眼睛看着都难受 --------------------编程问答-------------------- while (sdrEmp.Read())
  {
  this.lBoxEmp.Items.Add(new ListItem(sdrEmp.GetString(1), sdrEmp.GetInt32(0).ToString()));
  }
sdrEmp.Close();
 con.Close(); --------------------编程问答-------------------- 额，不会吧~ --------------------编程问答-------------------- while (sdrEmp.Read())
  {
  this.lBoxEmp.Items.Add(new ListItem(sdrEmp.GetString(1), sdrEmp.GetInt32(0).ToString()));
  sdrEmp.Close();
  //关闭连接
  con.Close();
  }

 

补充：.NET技术 ,  C#