Jeffrey Richter在techhed 2005上说:

"I have come to learn over the years the C# compiler is basically a liar to me." .. "It lies about many things." .. "Like when you do a foreach loop..." .. "...that is one little line of code that you write, but what the C# compiler spits out in order to do that it's phenomenal. It puts out a try/finally block in there, inside the finally block it casts your variable to an IDisposable interface, and if the cast suceeds it calls the Dispose method on it, inside the loop it calls the Current property and the MoveNext method repeatedly inside the loop, objects are being created underneath the covers. A lot of people use foreach because it's very easy coding, very easy to do.." .. "foreach is not very good in terms of performance, if you iterated over a collection instead by using square bracket notation, just doing index, that's just much faster, and it doesn't create any objects on the heap..."

按需网络直播: http://msevents.microsoft.com/CUI/WebCastEventDetails.aspx?EventID=1032292286&EventCategory=3&culture=en-US&CountryCode=US

首先是对德米特里的回答(现已删除)的反诉。对于数组，c#编译器为foreach生成的代码与等效的For循环生成的代码大致相同。这就解释了为什么这个基准测试的结果基本相同:

using System;
using System.Diagnostics;
using System.Linq;

class Test
{
    const int Size = 1000000;
    const int Iterations = 10000;

    static void Main()
    {
        double[] data = new double[Size];
        Random rng = new Random();
        for (int i=0; i < data.Length; i++)
        {
            data[i] = rng.NextDouble();
        }

        double correctSum = data.Sum();

        Stopwatch sw = Stopwatch.StartNew();
        for (int i=0; i < Iterations; i++)
        {
            double sum = 0;
            for (int j=0; j < data.Length; j++)
            {
                sum += data[j];
            }
            if (Math.Abs(sum-correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("For loop: {0}", sw.ElapsedMilliseconds);

        sw = Stopwatch.StartNew();
        for (int i=0; i < Iterations; i++)
        {
            double sum = 0;
            foreach (double d in data)
            {
                sum += d;
            }
            if (Math.Abs(sum-correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("Foreach loop: {0}", sw.ElapsedMilliseconds);
    }
}

结果:

For loop: 16638
Foreach loop: 16529

接下来，验证Greg关于集合类型的观点是重要的——在上面将数组更改为List<double>，您将得到完全不同的结果。它不仅在一般情况下要慢得多，而且foreach也比通过索引访问慢得多。话虽如此，我仍然几乎总是喜欢foreach而不是for循环，因为它使代码更简单——因为可读性几乎总是重要的，而微优化很少是。

这可能取决于您枚举的集合类型及其索引器的实现。一般来说，使用foreach可能是一种更好的方法。

而且，它可以与任何IEnumerable一起工作，而不仅仅是与索引器一起工作。

“有没有什么论据可以帮助我说服他使用for循环是可以接受的?”

不，如果你的老板事无巨细地告诉你应该使用哪种编程语言结构，你就真的没什么可说的了。对不起。

每当有关于性能的争论时，您只需要编写一个小测试，以便您可以使用量化结果来支持您的案例。

使用StopWatch类，为了精确起见，重复某件事几百万次。(如果没有for循环，这可能很难):

using System.Diagnostics;
//...
Stopwatch sw = new Stopwatch()
sw.Start()
for(int i = 0; i < 1000000;i ++)
{
    //do whatever it is you need to time
}
sw.Stop();
//print out sw.ElapsedMilliseconds

幸运的是，这样做的结果表明差异可以忽略不计，您还可以在最可维护的代码中执行任何结果

你可以在Deep .NET - part 1迭代中读到它

它覆盖了从。net源代码一直到反汇编的结果(没有第一次初始化)。

数组迭代使用foreach循环:

And - list迭代foreach循环:

最终结果是: