//开始修改时单元格内的值
        string preValue = "";
        private void dataGrid_BeginningEdit(object sender, DataGridBeginningEditEventArgs e)
        {
            //将修改前的值保存起来
            preValue = (e.Column.GetCellContent(e.Row) as TextBlock).Text;           
        }

        private void dataGrid_CellEditEnding(object sender, DataGridCellEditEndingEventArgs e)
        {
            string newValue = (e.EditingElement as TextBox).Text;          
            //如果修改后的值和修改前的值不一样
            if (preValue != newValue)
            {
                    //进一步操作
            }
        }