Foreach循环比for循环展示了更具体的意图。

使用foreach循环向使用您代码的任何人表明，您计划对集合中的每个成员执行一些操作，而不管其在集合中的位置。它还显示您没有修改原始集合(如果您试图修改，则会抛出异常)。

foreach的另一个优点是它适用于任何IEnumerable，而as for只适用于IList，其中每个元素实际上都有一个索引。

但是，如果需要使用元素的索引，那么当然应该允许使用for循环。但是如果您不需要使用索引，那么使用索引只会使您的代码变得混乱。

据我所知，这对性能没有重大影响。在未来的某个阶段，使用foreach调整代码以在多核上运行可能会更容易，但现在还不需要担心这一点。

Patrick Smacchia上个月写了一篇博客，总结如下:

List上的for循环比foreach便宜2倍多一点 循环列表。 在array上循环比在List上循环便宜2倍左右。 因此，使用for对数组进行循环要便宜5倍 而不是使用foreach在List上循环 (我相信，这是我们都在做的事情)。

    internal static void Test()
    {
        int LOOP_LENGTH = 10000000;
        Random random = new Random((int)DateTime.Now.ToFileTime());

        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                for (int k = 0; k < dict.Count; k++)
                {
                    if (dict[k] > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary for T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict)
                {
                    if (item.Value > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary foreach T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }

        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict.Values)
                {
                    if (item > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary foreach values T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            List<int> dict = new List<int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                for (int k = 0; k < dict.Count; k++)
                {
                    if (dict[k] > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"list for T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            List<int> dict = new List<int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict)
                {
                    if (item > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"list foreach T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }
    }

T:10.1957728s M:2080的字典 字典T:10.5900586 M:1952 字典foreach值T:3.8294776s M:2088 T:3.7981471s M:320 T:4.4861377s M:648

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

完全禁止使用for循环之类的东西似乎有点奇怪。

这里有一篇有趣的文章，介绍了两个循环之间的许多性能差异。

我个人会说，我发现foreach比for循环更具可读性，但你应该使用最好的方法来完成手头的工作，如果for循环更合适的话，就不必编写额外的长代码来包含foreach循环。

我遇到了一个案子，foreach比For快得多

为什么foreach在读取richtextbox行时比for循环快

我有一个类似于那个问题中的OP的案例。

A textbox reading about 72K lines, and I was accessling the Lines property(which is actually a getter method). (And apparently often in winforms there are getter methods that aren't O(1). I suppose it's O(n), so the larger the textbox the longer it takes to get a value from that 'property'. And in the for loop I had as the OP there had for(int i=0;i<textBox1.lines.length;i++) str=textBox1.Lines[i] , and it was really quite slow as it was reading the entire textbox each time it read a line plus it was reading the entire textbox each time it checked the condition.

Jon Skeet演示了您可以只访问一次Lines属性(甚至不是每次迭代一次，只是一次)。而不是每次迭代两次(这是大量的次数)。Do string[] strarlines = textBox1.Lines;然后在星线间循环。

但是一个直观形式的for循环访问Lines属性是非常低效的

for (int i = 0; i < richTextBox.Lines.Length; i++)
{
    s = richTextBox.Lines[i];
}

对于文本框，或者富文本框，它非常慢。

OP在一个富文本框上测试了这个循环，发现“有15000行。For循环花了8分钟才循环到15000行。而foreach只花了不到一秒钟的时间来列举它。”

那个链接的OP发现这个foreach比上面提到的他(同一个OP)的for循环要有效得多。就像我一样。

   String s=String.Empty;
   foreach(string str in txtText.Lines)
    {
       s=str;
    }