Question

Which query has better performance to return an object which has the max value for a specific property?

 var i = from item in listOfItems
         orderby item.Number descending
         select item;

or:

 var i = from item in listOfItems
          where item.Number== (from l in listOfItems select item).Max(l => l.Number)
          select item;

Answer 1

In SQL, the two queries are translated into (at least, if you do a .FirstOrDefault() in the resulting IEnumerables to select the wanted object):

SELECT TOP (1) [t0].[Number]
FROM [Item] AS [t0]
ORDER BY [t0].[Number] DESC

SELECT TOP (1) [t0].[Number]
FROM [Item] AS [t0]
WHERE [t0].[Number] = ((
    SELECT MAX([t1].[Number])
    FROM [Item] AS [t1]
    ))

I can’t say which would be faster based on anything factual, but I’d go with the first order-by solution.

-* Edited to address querying the database *-

If you are querying an in memory collection, the difference should be very small.

However, here is the results of an entirely unscientific test by querying 100.000 semirandom numbers in the two different ways of querying each, 10 times:

By desc (0): 00:00:00.0173879
By .Max (0): 00:00:00.0132833

By desc (1): 00:00:00.0250781
By .Max (1): 00:00:00.0140374

By desc (2): 00:00:00.0073955
By .Max (2): 00:00:00.0111658

By desc (3): 00:00:00.0066200
By .Max (3): 00:00:00.0115127

By desc (4): 00:00:00.0071220
By .Max (4): 00:00:00.0119572

By desc (5): 00:00:00.0070341
By .Max (5): 00:00:00.0114320

By desc (6): 00:00:00.0066670
By .Max (6): 00:00:00.0111127

By desc (7): 00:00:00.0071905
By .Max (7): 00:00:00.0116715

By desc (8): 00:00:00.0065414
By .Max (8): 00:00:00.0118076

By desc (9): 00:00:00.0071662
By .Max (9): 00:00:00.0131962

Here is the entire LINQPad script:

void Main()
{
    var listOfItems = new List<Item>();

    // Make 100000 Items with semirandom numbers
    for(int i=0; i<100000; i++)
    {
        listOfItems.Add(new Item { Number = i * DateTime.Now.Ticks });
    }

    for(int i=0; i<10; i++)
    {
        RunTest(i, listOfItems);
    }
}

void RunTest(int count, List<Item> listOfItems)
{
    var timer = new System.Diagnostics.Stopwatch();
    timer.Start();
    // Find by descending
    for(int i=0; i<100000; i++)
    {
        var itemWithLargestNumber = 
            from item in listOfItems
            orderby item.Number descending
            select item;
    }
    timer.Stop();
    string.Format("By desc ({0}): {1}", count, timer.Elapsed).Dump();

    timer = new System.Diagnostics.Stopwatch();
    timer.Start();

    // Find by .Max
    for(int i=0; i<100000; i++)
    {
        var itemWithLargestNumber = 
            from item in listOfItems
            where item.Number== (from l in listOfItems select item).Max(l => l.Number)
            select item;
    }
    timer.Stop();
    string.Format("By .Max ({0}): {1}", count, timer.Elapsed).Dump();

    "".Dump();
}

class Item 
{
    public long Number { get; set; }
}