private void btnChon_Click(object sender, EventArgs e)
        {
            bool Chon = false;

            try
            {
                foreach (DataRow dr in dtLop.Rows)
                {
                    if (bool.Parse(dr["Chon"].ToString()) == true)
                    {
                        Chon = true;
                        pKQHT_CTDT_LopInfo.IDDM_Lop = int.Parse(dr["DM_LopID"].ToString());
                        oBKQHT_CTDT_Lop.Add(pKQHT_CTDT_LopInfo);
                    }
                }
                if (Chon == false)
                {
                    ThongBao("Bạn chưa chọn lớp nào.");
                }
                else
                {
                    this.Tag = "1";
                    this.Close();
                }
            }
            catch (Exception ex)
            {
                ThongBao(ex.Message);
            }
        }