C#List Comprehensions =纯粹的句法糖？

Question

考虑以下C#代码:

IEnumerable numbers = Enumerable.Range(0, 10);
var evens = from num in numbers where num % 2 == 0 select num;

这是一个纯粹的语法糖,允许我写一个for或foreach循环作为一个班轮？是否有任何编译器优化使得上面的列表理解比循环结构更有效？这是如何工作的？

Answer 1

正如杰森所说,你的代码相当于:

Enumerable.Range(0, 10).Where(n => n % 2 == 0);

请注意,lambda将转换为函数调用,该函数调用是针对每个元素完成的.这可能是开销的最大部分.我做了一个测试,这表明LINQ在这个确切的任务上慢了3倍(mono gmcs version 1.2.6.0)

    Time for 10000000 for loop reps: 00:00:17.6852560
    Time for 10000000 LINQ reps: 00:00:59.0574430

    Time for 1000000 for loop reps: 00:00:01.7671640
    Time for 1000000 LINQ reps: 00:00:05.8868350

编辑:Gishu报告VS2008和框架v3.5 SP1给出:

    Time for 1000000 loop reps: :00.3724585 
    Time for 1000000 LINQ reps: :00.5119530 

LINQ大约慢了1.4倍.

它将for循环和列表与LINQ(以及它在内部使用的任何结构)进行比较.无论哪种方式,它将结果转换为数组(迫使LINQ停止"懒惰").两个版本重复:

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

public class Evens
{
    private static readonly int[] numbers = new int[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

    private static int MAX_REPS = 1000000;

    public static void Main()
    {
        Stopwatch watch = new Stopwatch();

        watch.Start();
        for(int reps = 0; reps < MAX_REPS; reps++)
        {
            List<int> list = new List<int>(); // This could be optimized with a default size, but we'll skip that.
            for(int i = 0; i < numbers.Length; i++)
            {
                int number = numbers[i];
                if(number % 2 == 0)
                    list.Add(number);
            }
            int[] evensArray = list.ToArray();
        }
        watch.Stop();
        Console.WriteLine("Time for {0} for loop reps: {1}", MAX_REPS, watch.Elapsed);

        watch.Reset();
        watch.Start();
        for(int reps = 0; reps < MAX_REPS; reps++)
        {
            var evens = from num in numbers where num % 2 == 0 select num;
            int[] evensArray = evens.ToArray();
        }
        watch.Stop();
        Console.WriteLine("Time for {0} LINQ reps: {1}", MAX_REPS, watch.Elapsed);
    }
}

过去对类似任务的性能测试(例如LINQ vs Loop - 性能测试)证实了这一点.

Answer 2

您可以进一步简化代码

var evens = Enumerable.Range(0, 10).Where(n => n % 2 == 0);

此形式的一个优点是延迟执行此表达式,直到evens迭代遍历(foreach(var n in evens) { ... }).上面的语句只是告诉编译器捕获如何枚举0到10之间的偶数的想法,但是在绝对必要之前不执行该想法.