//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
            }
        }