.net - .NET 4.0 中的 Array.Sort() 发生了什么？ TrySZSort() 消失了吗？

Question

我想知道为什么下面的代码片段没有给出预期的结果？它对一个不太小的随机数组进行排序，并使用 3 种不同的方法。我预计速度会是这样:

1. Array.Sort() - 使用 native TrySZSort 函数最快，我记得在 .NET 2.0 中
2. 使用自定义 Comparer 类进行降序排序
3. lambda 表达式排序。

代码:

class DescComparer : IComparer<double> {
    // simple comparison 
    // (yes, its not exactly correct ...)
    public int Compare(double x, double y) {
        return (x > y) ? -1 : 1; 
    }
}
static void Main(string[] args) {
    Stopwatch sw = new Stopwatch(); 
    Random rand = new Random(); 
    DescComparer comparer = new DescComparer();
    double[] a = new double[1000000]; 
    for (int r = 0; r < 20; r++) {

        // init array
        for (int i = 0; i < a.Length; i++) a[i] = rand.NextDouble();  
        sw.Restart();
        Array.Sort(a);
        sw.Stop();
        Console.WriteLine("ascending took: {0} ms ", sw.ElapsedMilliseconds);

        // init array
        for (int i = 0; i < a.Length; i++) a[i] = rand.NextDouble();
        sw.Restart();
        Array.Sort<double>(a, comparer);
        sw.Stop();
        Console.WriteLine("descending took: {0} ms ", sw.ElapsedMilliseconds);

        // init array
        for (int i = 0; i < a.Length; i++) a[i] = rand.NextDouble();
        sw.Restart();
        Array.Sort<double>(a, (x,y) => -x.CompareTo(y));
        sw.Stop();
        Console.WriteLine("desc lambda took: {0} ms ", sw.ElapsedMilliseconds);

    }
    Console.Read(); 
}

但奇怪的是，它给出了以下内容:

ascending took: 514 ms
descending took: 537 ms
desc lambda took: 915 ms
ascending took: 511 ms
descending took: 492 ms
desc lambda took: 923 ms
ascending took: 511 ms
descending took: 483 ms
desc lambda took: 912 ms
ascending took: 511 ms
descending took: 485 ms
desc lambda took: 914 ms
ascending took: 518 ms
descending took: 485 ms
desc lambda took: 924 ms
... a.s.o. ...

所以，lambda 确实是最慢的。但为什么普通的升序 Array.Sort 不再更快了呢？是因为 Array.Sort(T[], Comparer) 已得到改进，还是 TrySZSort 只是被删除了？或者我错过了什么？

(发布版本，无调试，目前没有可用的反射器；))谢谢!

更新:根据@Reed Copsey 的提示，lambda 表达式是不公平的。我尝试将其更改为与比较器相同的值。速度加快了。 Asc/lambda 从 55% -> 75%。所以它仍然慢得多。

Answer 1

So, the lambda really is slowest. But how comes, the plain ascending Array.Sort is not faster anymore? Is it, because Array.Sort(T[], Comparer) has been improved or has TrySZSort simply been removed? Or did I miss something?

嗯，这里有两个问题。首先，这实际上取决于您的构建和系统 -

在我的 x64 系统上，Array.Sort() 是最快的，而且速度快很多:

ascending took: 192 ms
descending took: 248 ms
desc lambda took: 326 ms
ascending took: 194 ms
descending took: 247 ms
desc lambda took: 326 ms

在 x86 上，情况略有不同 - 但与您所显示的意义并不相同:

ascending took: 235 ms
descending took: 223 ms
desc lambda took: 325 ms
ascending took: 234 ms
descending took: 222 ms
desc lambda took: 325 ms

当你运行这些计时时，你是否连接了 Visual Studio 主机？如果您在 VS 中运行(即:默认情况下使用 F5 而不是 Ctrl+F5)，即使是发布版本也会显着变慢。

<小时/>

另请注意，就 lambda 而言，您的测试并不完全公平。您应该使用相同的测试机制，即:

Array.Sort<double>(a, (x, y) => (x > y) ? -1 : 1);