WebForm获取选中行数值的问题

开发环境：
VS2010,WebForm,.NET 4.0

控件：
GridView+Button

需求：
GridView 展示多行数据， 用户选择其中一行，点击Button，将选中行的其中2列数据写入新表；

问题：
写入新表后， 发现选中数据始终为空值；

主要代码：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

namespace WebIMS.MeetingRoom
{
    public partial class Frm_Meeting_Order : System.Web.UI.Page
    {
        public string select_room_name = "";
        public string select_room_adds = "";
    ..............
..................
        protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
        {
            int itmpid = int.Parse(Session["tmpID"].ToString());
            select_room_name = GridView1.Rows[itmpid].Cells[1].Text;//读取Gridview里所选行的第1列
            select_room_adds = GridView1.Rows[itmpid].Cells[3].Text;//读取Gridview里所选行的第3列
            Notice0.Text = select_room_name;//选中时select_room_name是有值的；
            Session.Remove("tmpID"); //清除Session
   
        }

    ..................
.................
string strCmd = string.Format("insert into ");
            strCmd += ("Meeting_Order(Room_Name,Order_Date,Time_Start,Time_End,Subject,Remark,Number_People,Time_Created,Name_Created,Name_ID_Created,Tel_Created,Email_Created) ");
            strCmd += ("values('" + select_room_name + "','" + Order_Date + "'");//但这时select_room_name的值不知道为什么变成空值了？？；
            strCmd += (",'" + startTime + "','" + endTime + "','" + Subject.Text.Trim() + "','" + Remark.Text.Trim() + "' ");
            strCmd += (",'" + Number_People.Text.Trim() + "','" + DateTime.Now.ToString() + "','" + Session["Str_Name"] + "','" + Session["Str_ID"] + "'");
            strCmd += (",'" + Session["Str_Tel"] + "','" + Session["Str_Email"] + "' "); //创建人电话，Email
            strCmd += (")"); //SQL 语句结束

            try
            {
                SqlConnection sqlConn_insert = new SqlConnection(DBHelper.constr); //准备写入记录
                sqlConn_insert.Open();
                SqlCommand sqlCmd = new SqlCommand(strCmd, sqlConn_insert);

                if (sqlCmd.ExecuteNonQuery() > 0) //写入记录
                {
                    //新增成功!
               
                    Notice.Text = "会议室预约成功！会议室是：" + select_room_name;
                    
                    sqlConn_insert.Close();
..................
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