先Where 再First ,还是直接First.

    private static void FirstTest()
    {

        IEnumerable<Item> items = new List<Item>(Enumerable.Range(0, 1000).Select(a => new Item(a)));

        // sorting randomly
        items = items.OrderBy(x => new Guid()).ToArray();

        int n = 100;

        Func<Item, bool> filter = x => x.Size == n;

        Run("First", () => items.First(filter));
        Run("Where First", () => items.Where(filter).First());

        filter = x => x.Size < n;

        Run("First", () => items.First(filter));
        Run("Where First", () => items.Where(filter).First());

        filter = x => x.Size > n;

        Run("First", () => items.First(filter));
        Run("Where First", () => items.Where(filter).First());
    }

    public class Item
    {
        public String Name { get; set; }
        public int Size { get; set; }

        public Item(int n)
        {
            Size = n;
            Name = "a" + Size;
        }
    }


    public static void Run(String title, Func<object> action)
    {

        Stopwatch watch = Stopwatch.StartNew();

        for (int i = 0; i < 40000; i++)
        {
            action();
        }

        watch.Stop();


        Console.WriteLine("{0} milliseconds for {1}", watch.ElapsedMilliseconds, title);

    }

输出结果：