ASP.Net C# รบกสนถามเกี่ยวกับ DropdownList หน่อยครับตอนเลือกข้อมูล

public partial class It_Admin_Import : System.Web.UI.Page
    {

        private string AD_Host = ConfigurationManager.AppSettings["AD_Host"].ToString();
        private string AD_Port = ConfigurationManager.AppSettings["AD_Port"].ToString();
        private string AD_UserObjectClass = ConfigurationManager.AppSettings["AD_UserObjectClass"].ToString();
        private string AD_authenticationWith = ConfigurationManager.AppSettings["AD_authenticationWith"].ToString();
        private string AD_Base = ConfigurationManager.AppSettings["AD_Base"].ToString();
        private string AD_Username = ConfigurationManager.AppSettings["AD_Username"].ToString();
        private string AD_Password = ConfigurationManager.AppSettings["AD_Password"].ToString();

        //clsDatabase clsDB = new clsDatabase();

        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void ddlSelect_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (ddlSelect.SelectedValue == "01")
            {
                pnlImport.Visible = true;
                pnlExport.Visible = false;


            }
            else if (ddlSelect.SelectedValue == "02")
            {
                pnlImport.Visible = false;
                pnlExport.Visible = true;
            }
            else
            {
                Page.ClientScript.RegisterStartupScript(this.GetType(), "ErrorAlert", "alert('กรุณาเลือกประเภทข้อมูลที่ต้องการโชว์');", true);
                pnlImport.Visible = false;
                pnlExport.Visible = false;
            }
        }

        protected void ddlSearch_Type_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (ddlSearch_Type.SelectedValue == "01")
            {
                gvStaff.Visible = true;
                gvStudent.Visible = false;

                BindData_gvStaff();
            }
            else if (ddlSearch_Type.SelectedValue == "02")
            {
                gvStaff.Visible = false;
                gvStudent.Visible = true;

                BindData_gvStudent();
            }
            else
            {
                Page.ClientScript.RegisterStartupScript(this.GetType(), "ErrorAlert", "alert('กรุณาเลือกข้อมูลที่ต้องการ Export');", true);
                gvStaff.Visible = false;
                gvStudent.Visible = false;
            }
        }

        protected void btnSearch_Click(object sender, EventArgs e)
        {
            // Search STAFF && STUDENT //
            String strSQL;
            DataTable dt;
            
            if (txtSearch.Text.Length > 0)
            {
                if (ddlSearch_Type.SelectedValue == "01")
                {
                    clsDatabase clsDB = new clsDatabase("0");
                    strSQL = "SELECT CITIZEN_ID,STF_FNAME,STF_LNAME,BIRTHDAY,TELEPHONE FROM UOC_STAFF WHERE CITIZEN_ID = '" + this.txtSearch.Text + "' ";
                    dt = clsDB.QueryDataTable(strSQL);
                    ViewState["ds"] = dt;
                    if (dt.Rows.Count > 0)
                    {
                        gvStaff.Visible = true;
                        gvStudent.Visible = false;
                        gvStaff.DataSource = dt;
                        gvStaff.DataBind();
                    }
                }
                else if (ddlSearch_Type.SelectedValue == "02")
                {
                    clsDatabase clsDB = new clsDatabase("1");
                    strSQL = "SELECT STUDENTCODE,CITIZEN_ID,STD_FNAME,STD_LNAME,BIRTHDAY FROM UOC_STUDENT WHERE CITIZEN_ID = '" + this.txtSearch.Text + "' ";
                    dt = clsDB.QueryDataTable(strSQL);
                    ViewState["ds"] = dt;
                    if(dt.Rows.Count > 0)
                    {
                        gvStaff.Visible = false;
                        gvStudent.Visible = true;
                        gvStudent.DataSource = dt;
                        gvStudent.DataBind();
                    }
                }
            }

        }

        protected void BindData_gvStaff()
        {
            String strSQL;
            clsDatabase clsDB = new clsDatabase("0"); 
            strSQL = "SELECT CITIZEN_ID,STF_FNAME,STF_LNAME,BIRTHDAY,TELEPHONE FROM UOC_STAFF";

            ViewState["ds"] = clsDB.QueryDataTable(strSQL);
            gvStaff.DataSource = clsDB.QueryDataTable(strSQL);
            gvStaff.DataBind();
        }
        
        protected void BindData_gvStudent()
        {
            String strSQL;
            clsDatabase clsDB = new clsDatabase("1"); 
            strSQL = "SELECT STUDENTCODE,CITIZEN_ID,STD_FNAME,STD_LNAME,BIRTHDAY FROM UOC_STUDENT";

            ViewState["ds"]  = clsDB.QueryDataTable(strSQL);
            gvStudent.DataSource = clsDB.QueryDataTable(strSQL);
            gvStudent.DataBind();

        }

        protected void gvStaff_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            String strSQL;
            clsDatabase clsDB = new clsDatabase("0"); 
            strSQL = "SELECT CITIZEN_ID,STF_FNAME,STF_LNAME,BIRTHDAY,TELEPHONE FROM UOC_STAFF";

            ViewState["ds"] = clsDB.QueryDataTable(strSQL);
            gvStaff.PageIndex = e.NewPageIndex;
            gvStaff.DataSource = clsDB.QueryDataTable(strSQL);
            gvStaff.DataBind();
        }

        protected void gvStudent_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            String strSQL;
            clsDatabase clsDB = new clsDatabase("1"); 
            strSQL = "SELECT STUDENTCODE,CITIZEN_ID,STD_FNAME,STD_LNAME,BIRTHDAY FROM UOC_STUDENT";

            ViewState["ds"]  = clsDB.QueryDataTable(strSQL);
            gvStudent.PageIndex = e.NewPageIndex;
            gvStudent.DataSource = clsDB.QueryDataTable(strSQL);
            gvStudent.DataBind();
        }

        protected void btnExport_Click(object sender, EventArgs e)
        {
            
            if (ddlSearch_Type.SelectedValue == "01")
            {
                gvStaff.DataBind();
                string filename = "Staff.xls";
                System.IO.StringWriter tw = new System.IO.StringWriter();
                System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
                DataGrid dgGrid = new DataGrid();

                dgGrid.DataSource = ViewState["ds"];
                dgGrid.DataBind();

                //Get the HTML for the control.
                dgGrid.RenderControl(hw);
                //Write the HTML back to the browser.
                //Response.ContentType = application/vnd.ms-excel;
                Response.ContentType = "application/vnd.ms-excel";
                Response.AppendHeader("Content-Disposition", "attachment; filename=" + filename + "");
                this.EnableViewState = false;
                Response.Write(tw.ToString());
                Response.End();
            }
            else if (ddlSearch_Type.SelectedValue == "02")
            {
                gvStudent.DataBind();
                string filename = "Student.xls";
                System.IO.StringWriter tw = new System.IO.StringWriter();
                System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
                DataGrid dgGrid = new DataGrid();

                dgGrid.DataSource = ViewState["ds"];
                dgGrid.DataBind();

                //Get the HTML for the control.
                dgGrid.RenderControl(hw);
                //Write the HTML back to the browser.
                //Response.ContentType = application/vnd.ms-excel;
                Response.ContentType = "application/vnd.ms-excel";
                Response.AppendHeader("Content-Disposition", "attachment; filename=" + filename + "");
                this.EnableViewState = false;
                Response.Write(tw.ToString());
                Response.End();
            }
            else
            {