lambda函数比委托/匿名函数更快吗？

Question

我假设lambda functions,delegates并且anonymous functions使用相同的主体将具有相同的"速度",但是,运行以下简单程序:

static void Main(string[] args)
{
    List<int> items = new List<int>();

    Random random = new Random();

    for (int i = 0; i < 10000000; i++)
    {
        items.Add(random.Next());
    }

    Stopwatch watch;
    IEnumerable<int> result;

    Func<int, bool> @delegate = delegate(int i)
    {
        return i < 500;
    };
    watch = Stopwatch.StartNew();
    result = items.Where(@delegate);
    watch.Stop();
    Console.WriteLine("Delegate: {0}", watch.Elapsed.TotalMilliseconds);

    Func<int, bool> lambda = i => i < 500;
    watch = Stopwatch.StartNew();
    result = items.Where(lambda);
    watch.Stop();
    Console.WriteLine("Lambda: {0}", watch.Elapsed.TotalMilliseconds);

    watch = Stopwatch.StartNew();
    result = items.Where(i => i < 500);
    watch.Stop();
    Console.WriteLine("Inline: {0}", watch.Elapsed.TotalMilliseconds);

    Console.ReadLine();
}

我明白了:

代表:4.2948毫秒

Lambda:0.0019毫秒

匿名:0.0034毫秒

虽然可以忽略不计,为什么这三种 - 显然是相同的 - 方法以不同的速度运行？引擎盖下发生了什么？

---

更新:

正如评论所建议的那样,以下"强制" Where通过呼吁ToList()它.此外,还添加了一个循环以提供更多运行数据:

while (true) 
{
    List<int> items = new List<int>();

    Random random = new Random();

    for (int i = 0; i < 10000000; i++)
    {
        items.Add(random.Next());
    }

    Stopwatch watch;
    IEnumerable<int> result;

    Func<int, bool> @delegate = delegate(int i)
    {
        return i < 500;
    };
    watch = Stopwatch.StartNew();
    result = items.Where(@delegate).ToList();
    watch.Stop();
    Console.WriteLine("Delegate: {0}", watch.Elapsed.TotalMilliseconds);

    Func<int, bool> lambda = i => i < 500;
    watch = Stopwatch.StartNew();
    result = items.Where(lambda).ToList();
    watch.Stop();
    Console.WriteLine("Lambda: {0}", watch.Elapsed.TotalMilliseconds);

    watch = Stopwatch.StartNew();
    result = items.Where(i => i < 500).ToList();
    watch.Stop();
    Console.WriteLine("Inline: {0}", watch.Elapsed.TotalMilliseconds);
    Console.WriteLine(new string('-', 12));

}

上述代码每个函数的结果约为120毫秒.

Answer 1

其他人的结果表明性能是相同的:

http://blogs.microsoft.co.il/blogs/alex_golesh/archive/2007/12/11/anonymous-delegates-vs-lambda-expressions-vs-function-calls-performance.aspx

正如评论中所指出的,微观基准往往具有误导性.有太多因素无法控制,JIT优化,垃圾收集周期等......

看到这个相关的问题:

何时不使用lambda表达式

最后,我认为你的测试存在根本缺陷!您使用Linq Where扩展方法来执行代码.但是,Linq使用延迟评估,只有在开始迭代结果时才会执行代码!

Answer 2

lambda表达式是一个匿名函数."匿名函数"指的是lambda表达式或匿名方法(这就是你所谓的代码中的"代表").

这三个操作都使用了代理.第二个和第三个都使用lambda表达式.所有三个都将以相同的方式执行,具有相同的性能特征.

请注意,有可能是在两者之间性能差异:

Func<int, int> func = x => ...;
for (int i = 0; i < 10000; i++) {
    CallFunc(func);
}

和

for (int i = 0; i < 10000; i++) {
    CallFunc(x => ...) // Same lambda as before
}

这取决于编译器是否能够缓存由lambda表达式创建的委托.这将取决于它是否捕获变量等.

例如,考虑以下代码:

using System;
using System.Diagnostics;

class Test
{
    const int Iterations = 1000000000;

    static void Main()
    {
        AllocateOnce();
        AllocateInLoop();
    }

    static void AllocateOnce()
    {
        int x = 10;

        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        Func<int, int> allocateOnce = y => y + x;
        for (int i = 0; i < Iterations; i++)
        {
            sum += Apply(i, allocateOnce);
        }
        sw.Stop();
        Console.WriteLine("Allocated once: {0}ms", sw.ElapsedMilliseconds);
    }

    static void AllocateInLoop()
    {
        int x = 10;

        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        for (int i = 0; i < Iterations; i++)
        {
            sum += Apply(i, y => y + x);
        }
        sw.Stop();
        Console.WriteLine("Allocated in loop: {0}ms", sw.ElapsedMilliseconds);
    }

    static int Apply(int loopCounter, Func<int, int> func)
    {
        return func(loopCounter);
    }
}

编译器很聪明,但仍然存在差异.使用Reflector,我们可以看到它AllocateInLoop被有效地编译为:

private static void AllocateInLoop()
{
    Func<int, int> func = null;
    int x = 10;
    Stopwatch stopwatch = Stopwatch.StartNew();
    int sum = 0;
    for (int i = 0; i < Iterations; i++)
    {
        if (func == null)
        {
            func = y => y + x;
        }
        sum += Apply(i, func);
    }
    stopwatch.Stop();
    Console.WriteLine("Allocated in loop: {0}ms", stopwatch.ElapsedMilliseconds);
}

所以仍然只创建了一个委托实例,但循环中还有额外的逻辑 - 基本上是对每次迭代进行额外的无效测试.

在我的机器上,性能差异大约为15%.