//重载_SortCompare方法
        private void DataGridView1_SortCompare(object sender,DataGridViewSortCompareEventArgs e)
        {
            //   如果是学号或成绩列，则按浮点数处理
            if(e.Column.Name=="学号"|| e.Column.Name=="成绩")
            {
                e.SortResult = (Convert.ToDouble(e.CellValue1) - Convert.ToDouble(e.CellValue2) > 0) ? 1 : (Convert.ToDouble(e.CellValue1) - Convert.ToDouble(e.CellValue2) < 0)?-1:0;
            }
            //否则，按字符串比较
            else
            {
                e.SortResult = System.String.Compare(Convert.ToString(e.CellValue1), Convert.ToString(e.CellValue2));
            }
           

            // 如果发现两行相同，则按学号排序  
            if (e.SortResult == 0 && e.Column.Name != "学号")
            {
                e.SortResult = Convert.ToInt32(DataGridView1.Rows[e.RowIndex1].Cells["学号"].Value.ToString()) -
                   &n