update ข้อมูลในgridviewไม่ได้ค่ะ แต่สามารถลบหรือเพิ่มข้อมูลได้ตามปกติ

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

namespace Dentist
{
    public partial class StaffDate : System.Web.UI.Page
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["Dentist_Pool"].ConnectionString);

        SqlDataAdapter da = null;
        DataTable dt = null;

        protected void Page_Load(object sender, EventArgs e)
        {

            con.Open();
            String sqlSelect2 = "select StaffdateId, StaffLeaveStart, StaffLeaveFinish, StaffTimeStart, StaffTimeFinish, StaffLeaveNote from Staffdate where StaffId = '" + Session["sessionStaffId"] + "'";
            da = new SqlDataAdapter(sqlSelect2, con);
            con.Close();

            if (!Page.IsPostBack)
            {
                BindData();
            }
            
            
        }

        void BindData()
        {
            dt = new DataTable();
            da.Fill(dt);
            grdStaffdate.DataSource = dt;
            grdStaffdate.DataBind();

            dt = null;
            con.Close();
        }

        void Page_Unload()
        {
            con.Close();
            con = null;
        }

        protected void modEditCommand(object sender, GridViewEditEventArgs e)
        {
            Label lblStaffLeaveStart = (sender as GridView).Rows[e.NewEditIndex].FindControl("lblStaffLeaveStart") as Label;
            Label lblStaffLeaveFinish = (sender as GridView).Rows[e.NewEditIndex].FindControl("lblStaffLeaveFinish") as Label;
            Label lblStaffTimeStart = (sender as GridView).Rows[e.NewEditIndex].FindControl("lblStaffTimeStart") as Label;
            Label lblStaffTimeFinish = (sender as GridView).Rows[e.NewEditIndex].FindControl("lblStaffTimeFinish") as Label;
            Label lblStaffLeaveNote = (sender as GridView).Rows[e.NewEditIndex].FindControl("lblStaffLeaveNote") as Label;

            grdStaffdate.EditIndex = e.NewEditIndex;
            grdStaffdate.ShowFooter = false;

            BindData();
            
            TextBox txtEditLeaveStart = (sender as GridView).Rows[e.NewEditIndex].FindControl("txtEditLeaveStart") as TextBox;
            TextBox txtEditLeaveFinish = (sender as GridView).Rows[e.NewEditIndex].FindControl("txtEditLeaveFinish") as TextBox;
            DropDownList ddlEditTimeStart = (sender as GridView).Rows[e.NewEditIndex].FindControl("ddlEditTimeStart") as DropDownList;
            DropDownList ddlEditTimeFinish = (sender as GridView).Rows[e.NewEditIndex].FindControl("ddlEditTimeFinish") as DropDownList;
            TextBox txtEditLeaveNote = (sender as GridView).Rows[e.NewEditIndex].FindControl("txtEditLeaveNote") as TextBox;

            txtEditLeaveStart.Text = lblStaffLeaveStart.Text;
            txtEditLeaveFinish.Text = lblStaffLeaveFinish.Text;
            ddlEditTimeStart.Text = lblStaffTimeStart.Text;
            ddlEditTimeFinish.Text = lblStaffTimeFinish.Text;
            txtEditLeaveNote.Text = lblStaffLeaveNote.Text;
        }
        protected void modCancelCommand(object sender, GridViewCancelEditEventArgs e)
        {
            grdStaffdate.EditIndex = -1;
            grdStaffdate.ShowFooter = true;

            BindData();
        }
        protected void modDeleteCommand(object sender, GridViewDeleteEventArgs e)
        {
            con.Open();
            String sqlDelete = "delete from Staffdate where StaffdateId = '" + grdStaffdate.DataKeys[e.RowIndex].Value + "'";
            SqlCommand com = new SqlCommand(sqlDelete, con);
            com.ExecuteNonQuery();
            con.Close();

            grdStaffdate.EditIndex = -1;

            BindData();
        }
        protected void modUpdateCommand(object sender, GridViewUpdateEventArgs e)
        {

            TextBox txtEditLeaveStart = (TextBox)grdStaffdate.Rows[e.RowIndex].FindControl("txtEditLeaveStart");
            TextBox txtEditLeaveFinish = (TextBox)grdStaffdate.Rows[e.RowIndex].FindControl("txtEditLeaveFinish");
            DropDownList ddlEditTimeStart = (DropDownList)grdStaffdate.Rows[e.RowIndex].FindControl("ddlEditTimeStart");
            DropDownList ddlEditTimeFinish = (DropDownList)grdStaffdate.Rows[e.RowIndex].FindControl("ddlEditTimeFinish");
            TextBox txtEditLeaveNote = (TextBox)grdStaffdate.Rows[e.RowIndex].FindControl("txtEditLeaveNote");

            con.Open();
            String sqlUpdate = "update Staffdate set StaffLeaveStart = '" + txtEditLeaveStart.Text + "'" + " , StaffLeaveFinish = '" + txtEditLeaveFinish.Text + "'" + " , StaffTimeStart = '" + ddlEditTimeStart.Text + "'"
                + " , StaffTimeFinish ='" + ddlEditTimeFinish.Text + "'" + " , StaffLeaveNote = '" + txtEditLeaveNote.Text + "'" + " where StaffId = '" + grdStaffdate.DataKeys[e.RowIndex].Value + "'";
            SqlCommand com = new SqlCommand(sqlUpdate, con);

            con.Close();

            grdStaffdate.EditIndex = -1;
            grdStaffdate.ShowFooter = true;
            BindData();

        }
        protected void modRowCommand(object sender, GridViewCommandEventArgs e)
        {

            if (e.CommandName == "Add")
            {
                TextBox txtAddLeaveStart = (TextBox)grdStaffdate.FooterRow.FindControl("txtAddLeaveStart");
                TextBox txtAddLeaveFinish = (TextBox)grdStaffdate.FooterRow.FindControl("txtAddLeaveFinish");
                DropDownList ddlAddTimeStart = (DropDownList)grdStaffdate.FooterRow.FindControl("ddlAddTimeStart");
                DropDownList ddlAddTimeFinish = (DropDownList)grdStaffdate.FooterRow.FindControl("ddlAddTimeFinish");
                TextBox txtAddLeaveNote = (TextBox)grdStaffdate.FooterRow.FindControl("txtAddLeaveNote");

                con.Open();
                String sqlInsert = "insert into Staffdate (StaffId, StaffLeaveStart, StaffLeaveFinish, StaffTimeStart, StaffTimeFinish, StaffLeaveNote)" +
                    "values ('" + Session["sessionStaffId"] + "','" + txtAddLeaveStart.Text + "','" + txtAddLeaveFinish.Text + "','" + ddlAddTimeStart.Text + "','" + ddlAddTimeFinish.Text + "', '" + txtAddLeaveNote.Text + "')";
                SqlCommand com = new SqlCommand(sqlInsert, con);
                com.ExecuteNonQuery();
                con.Close();

                BindData();
            }

        }

        
       

    }
}