private void btnXoa_Click(object sender, EventArgs e)
        {
            DataRow row = dt.Select("STT =" + Convert.ToInt32(textBox21.Text)) [0];
            row.BeginEdit();
            row.Delete();
            row.EndEdit();
            SqlCommand cmddelete = new SqlCommand();
            cmddelete.Connection = con;
            cmddelete.CommandType = CommandType.Text;
            cmddelete.CommandText = "Delete from DanhSach where STT = @STT";
            cmddelete.Parameters.Add("@STT", SqlDbType.Int, 50, "STT");
            da.DeleteCommand = cmddelete;
            
            MessageBox.Show("Bạn Đã Xóa Thành Công!", "THÔNG BÁO", MessageBoxButtons.OK);
            da.Update(dt);
        }