private void btnAdd_Click(object sender, EventArgs e)
        {
            var addFrm = new AddReturnForm();

            addFrm.AddReturn += AddFrm_AddReturn;
            this.Hide();
            addFrm.ShowDialog();
            this.Show();
        }
        private void btnUpdate_Click(object sender, EventArgs e)
        {
            if (gvReturns.SelectedRows.Count > 0)
            {
                ReturnDTO re = new ReturnDTO();
                re.Returnid = (string)gvReturns.SelectedRows[0].Cells["maphieutra"].Value;
                re.Readerid = (string)gvReturns.SelectedRows[0].Cells["madocgia"].Value;
                re.Bookid   = (string)gvReturns.SelectedRows[0].Cells["masach"].Value;
                re.RtnDate  = (DateTime)gvReturns.SelectedRows[0].Cells["ngaytra"].Value;
                re.Forfeit  = float.Parse(gvReturns.SelectedRows[0].Cells["tienphat"].Value.ToString());

                var addFrm = new AddReturnForm();
                addFrm.update     = true;
                addFrm.returns    = re;
                addFrm.AddReturn += AddFrm_AddReturn;
                this.Hide();
                addFrm.ShowDialog();
                this.Show();
            }
            else
            {
                MessageBox.Show("Select a row to update, pls!");
            }
        }