What is the performance of the Last() extension method for List?

Question

I really like Last() and would use it all the time for Lists. But since it seems to be defined for IEnumerable, I gu

Answer 1

It contains an optimisation for anything that implements IList in which case it just looks up the item at length -1.

Keep in mind that the vast majority of stuff you will send in will implement IList

List 
int[] 

and so on ... all implement IList

For those who can not look at the code to confirm, you can confirm it using observation:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

namespace ConsoleApplication4 {
    class Program {

        static void Profile(string description, int iterations, Action func) {

            // clean up
            GC.Collect();
            GC.WaitForPendingFinalizers();
            GC.Collect();

            // warm up 
            func();

            var watch = Stopwatch.StartNew();
            for (int i = 0; i < iterations; i++) {
                func();
            }
            watch.Stop();
            Console.Write(description);
            Console.WriteLine(" Time Elapsed {0} ms", watch.ElapsedMilliseconds);
        }

        static void Main(string[] args) {
            int[] nums = Enumerable.Range(1, 1000000).ToArray();

            int a;

            Profile("Raw performance", 100000, () => { a = nums[nums.Length - 1];  });
            Profile("With Last", 100000, () => { a = nums.Last(); }); 

            Console.ReadKey();
        }


    }
}

Output:

Raw performance Time Elapsed 1 ms
With Last Time Elapsed 31 ms

So it's only 30 times slower and maintains that performance profile with whatever length list you have, which is nothing in the big scheme of things.