在c# / VB.NET/。哪个循环运行得更快，for还是foreach?
自从很久以前我读到for循环比foreach循环工作得快，我就认为它适用于所有集合、泛型集合、所有数组等。
我搜索了谷歌，找到了几篇文章，但大多数都是不确定的(阅读文章评论)，而且是开放式的。
理想的情况是列出每种情况以及最佳解决方案。
例如(这只是一个例子):
用于迭代1000+的数组 字符串- for比foreach好 对于迭代IList(非泛型)字符串- foreach更好 比
在网上找到了一些相同的参考资料:
由Emmanuel Schanzer撰写的原创文章 CodeProject FOREACH Vs. FOR 博客——去博客还是不去博客，这是个问题 ASP。NET论坛- NET 1.1 c# for vs foreach
(编辑)
除了可读性之外，我对事实和数据真的很感兴趣。在某些应用中，最后一英里的性能优化确实很重要。
当前回答
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。
其他回答
影响性能的是您在循环中所做的事情，而不是实际的循环构造(假设您的情况不平凡)。
Jeffrey Richter在最近的播客上谈到了for和foreach之间的性能差异:http://pixel8.infragistics.com/shows/everything.aspx#Episode:9317
至少我没有看到我的同事或上级这么说，考虑到for和foreach之间没有显著的速度差异，这是荒谬的。如果他要求在所有情况下都使用它，同样适用!
我发现foreach循环迭代列表更快。下面是我的测试结果。在下面的代码中，我分别迭代一个大小为100、10000和100000的数组，使用for和foreach循环来测量时间。
private static void MeasureTime()
{
var array = new int[10000];
var list = array.ToList();
Console.WriteLine("Array size: {0}", array.Length);
Console.WriteLine("Array For loop ......");
var stopWatch = Stopwatch.StartNew();
for (int i = 0; i < array.Length; i++)
{
Thread.Sleep(1);
}
stopWatch.Stop();
Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("Array Foreach loop ......");
var stopWatch1 = Stopwatch.StartNew();
foreach (var item in array)
{
Thread.Sleep(1);
}
stopWatch1.Stop();
Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch1.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("List For loop ......");
var stopWatch2 = Stopwatch.StartNew();
for (int i = 0; i < list.Count; i++)
{
Thread.Sleep(1);
}
stopWatch2.Stop();
Console.WriteLine("Time take to run the for loop is {0} millisecond", stopWatch2.ElapsedMilliseconds);
Console.WriteLine(" ");
Console.WriteLine("List Foreach loop ......");
var stopWatch3 = Stopwatch.StartNew();
foreach (var item in list)
{
Thread.Sleep(1);
}
stopWatch3.Stop();
Console.WriteLine("Time take to run the foreach loop is {0} millisecond", stopWatch3.ElapsedMilliseconds);
}
更新
在@jgauffin建议后，我使用了@johnskeet代码，发现使用数组的for循环比下面的更快，
Foreach循环与数组。 For带列表的循环。 Foreach循环与列表。
请看下面我的测试结果和代码，
private static void MeasureNewTime()
{
var data = new double[Size];
var rng = new Random();
for (int i = 0; i < data.Length; i++)
{
data[i] = rng.NextDouble();
}
Console.WriteLine("Lenght of array: {0}", data.Length);
Console.WriteLine("No. of iteration: {0}", Iterations);
Console.WriteLine(" ");
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 with Array: {0}", sw.ElapsedMilliseconds);
sw = Stopwatch.StartNew();
for (var 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 with Array: {0}", sw.ElapsedMilliseconds);
Console.WriteLine(" ");
var dataList = data.ToList();
sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
double sum = 0;
for (int j = 0; j < dataList.Count; j++)
{
sum += data[j];
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("For loop with List: {0}", sw.ElapsedMilliseconds);
sw = Stopwatch.StartNew();
for (int i = 0; i < Iterations; i++)
{
double sum = 0;
foreach (double d in dataList)
{
sum += d;
}
if (Math.Abs(sum - correctSum) > 0.1)
{
Console.WriteLine("Summation failed");
return;
}
}
sw.Stop();
Console.WriteLine("Foreach loop with List: {0}", sw.ElapsedMilliseconds);
}
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
