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performancelinq
Jessica
Jessica

Reputation: 2051

Which LINQ Query is quicker? Sort or select last?

I'm writing a LINQ query where I have to select the last result in a list.

Which is more efficient?

.OrderByDescending(x => x.Id).First();

or

.Last();

Upvotes: 0

Views: 44

Answers (1)

Harald Coppoolse
Harald Coppoolse

Reputation: 30464

Neither can be said to be more efficient. They do different things.

.Last();

This one returns the last element of a sequence (if there is one), without ordering

.OrderByDescending(x => x.Id).First();

This one returns you the element that would be last if you'd order in ascending order, if there is one. It will give be the same result as:

.OrderBy(x => x.Id).Last();



var sequence = {4, 1, 3}
var a = sequence.Last();                        // result: 3
var b = sequence.OrderByDescending(x => x.Id)   // intermediate result: 4, 3, 1
                .First();                       // final result: 4
var c = sequence.OrderBy(x => x.Id)             // intermediate result: 1, 3, 4
                .Last();                        // final result: 4

b is more efficient than c, because after ordering the enumerator only has to MoveNext to the first element, while for c the enumerator has to MoveNext(), until there are no more elements.

Upvotes: 1

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