OrderBy().Last() or OrderByDescending().First() performance

Question

I know that this probably is micro-optimization, but still I wonder if there is any difference in using

var lastObject = myList.OrderBy(item => item.Created).Last(); 

or

var lastObject = myList.OrderByDescending(item => item.Created).First(); 

I am looking for answers for Linq to objects and Linq to Entities.

Answer 1

Assuming that both ways of sorting take equal time (and that's a big 'if'), then the first method would have the extra cost of doing a .Last(), potentially requiring a full enumeration.

And that argument probably holds even stronger for an SQL oriented LINQ.

Answer 2

(my answer is about Linq to Objects, not Linq to Entities)

I don't think there's a big difference between the two instructions, this is clearly a case of micro-optimization. In both cases, the collection needs to be sorted, which usually means a complexity of O(n log n). But you can easily get the same result with a complexity of O(n), by enumerating the collection and keeping track of the min or max value. Jon Skeet provides an implementation in his MoreLinq project, in the form of a MaxBy extension method:

var lastObject = myList.MaxBy(item => item.Created);