//给datagridview添加一个事件

 private void datagridview1_CellEndEdit(object sender, DataGridViewCellEventArgs e)
        {

            string classNumber = this.datagridview1.Rows[e.RowIndex].Cells[e.ColumnIndex].Value.ToString();

            if (true)//不存在
            {
                MessageBox.Show("编号不存在！");
                this.datagridview1.CurrentCell = this.datagridview1.Rows[e.RowIndex].Cells[e.ColumnIndex];
                this.datagridview1.BeginEdit(true);//设该单元格为编辑状态
            }
        }