我经常遇到这样的情况:我想在声明查询的地方对查询进行求值。这通常是因为我需要对它进行多次迭代，计算成本很高。例如:
string raw = "...";
var lines = (from l in raw.Split('\n')
let ll = l.Trim()
where !string.IsNullOrEmpty(ll)
select ll).ToList();
这很好。但是如果我不打算修改结果，那么我也可以调用ToArray()而不是ToList()。
然而，我想知道ToArray()是否通过首先调用ToList()来实现，因此内存效率比只调用ToList()低。
我疯了吗?我是否应该调用ToArray() -在知道内存不会被分配两次的情况下安全可靠?
当前回答
老问题，但总是有新的提问者。
根据来源的System.Linq。可枚举，ToList只返回一个新的List(source)，而ToArray使用一个新的Buffer<T>(source).ToArray()返回一个T[]。
关于内存分配:
当运行在IEnumerable<T> only对象上时，ToArray确实比ToList多分配一次内存。但是在大多数情况下您不必关心它，因为GC将在需要时进行垃圾收集。
关于运行时效率:
那些质疑这个问题的人可以在自己的机器上运行下面的代码，然后就会得到答案。
class PersonC
{
public Guid uuid;
public string name;
public int age;
public bool sex;
public DateTime BirthDay;
public double weight;
}
struct PersonS
{
public Guid uuid;
public string name;
public int age;
public bool sex;
public DateTime BirthDay;
public double weight;
}
class PersonT<T> : IEnumerable<T>
{
private List<T> items;
public PersonT(IEnumerable<T> init)
{
items = new List<T>(init);
}
public IEnumerator<T> GetEnumerator() => items.GetEnumerator();
IEnumerator IEnumerable.GetEnumerator() => items.GetEnumerator();
}
private IEnumerable<PersonC> C(int count)
{
for (var i = 0; i < count; ++i)
{
var guid = Guid.NewGuid();
var guidBytes = guid.ToByteArray(); //16 bytes
yield return new PersonC
{
uuid = guid,
name = guid.ToString(),
age = guidBytes[0] ^ guidBytes[7],
sex = guidBytes[14] % 2 == 0,
BirthDay = DateTime.Now.AddDays(-guidBytes[11] * 18),
weight = guidBytes[12] * 100
};
}
}
private IEnumerable<PersonS> S(int count)
{
for (var i = 0; i < count; ++i)
{
var guid = Guid.NewGuid();
var guidBytes = guid.ToByteArray(); //16 bytes
yield return new PersonS
{
uuid = guid,
name = guid.ToString(),
age = guidBytes[0] ^ guidBytes[7],
sex = guidBytes[14] % 2 == 0,
BirthDay = DateTime.Now.AddDays(-guidBytes[11] * 18),
weight = guidBytes[12] * 100
};
}
}
private void MakeLog(string test, List<long> log) =>
Console.WriteLine("{0} {1} ms -> [{2}]",
test,
log.Average(),
string.Join(", ", log)
);
private void Test1(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
MakeLog("C.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = C(count).ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = C(count).ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = S(count).ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = S(count).ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void Test2(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
var dataC1 = new PersonT<PersonC>(C(count));
var dataS1 = new PersonT<PersonS>(S(count));
MakeLog("C1.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC1.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C1.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC1.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S1.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS1.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S1.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS1.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void Test3(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
var dataC2 = (ICollection<PersonC>) new List<PersonC>(C(count));
var dataS2 = (ICollection<PersonS>) new List<PersonS>(S(count));
MakeLog("C2.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC2.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C2.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC2.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S2.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS2.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S2.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS2.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void TestMain()
{
const int times = 100;
const int count = 1_000_000 + 1;
Test1(times, count);
Test2(times, count);
Test3(times, count);
}
我在我的机器上得到了这些结果:
Group1:
C.ToList 761.79 ms -> [775, 755, 759, 759, 756, 759, 765, 750, 757, 762, 759, 754, 757, 753, 763, 753, 759, 756, 768, 754, 763, 757, 757, 777, 780, 758, 754, 758, 762, 754, 758, 757, 763, 758, 760, 754, 761, 755, 764, 847, 952, 755, 747, 763, 760, 758, 754, 763, 761, 758, 750, 764, 757, 763, 762, 756, 753, 759, 759, 757, 758, 779, 765, 760, 760, 756, 760, 756, 755, 764, 759, 753, 757, 760, 752, 764, 758, 760, 758, 760, 755, 761, 751, 753, 761, 762, 761, 758, 759, 752, 765, 756, 760, 755, 757, 753, 760, 751, 755, 779]
C.ToArray 782.56 ms -> [783, 774, 771, 771, 773, 774, 775, 775, 772, 770, 771, 774, 771, 1023, 975, 772, 767, 776, 771, 779, 772, 779, 775, 771, 775, 773, 775, 771, 765, 774, 770, 781, 772, 771, 781, 762, 817, 770, 775, 779, 769, 774, 763, 775, 777, 769, 777, 772, 775, 778, 775, 771, 770, 774, 772, 769, 772, 769, 774, 775, 768, 775, 769, 774, 771, 776, 774, 773, 778, 769, 778, 767, 770, 787, 783, 779, 771, 768, 805, 780, 779, 767, 773, 771, 773, 785, 1044, 853, 775, 774, 775, 771, 770, 769, 770, 776, 770, 780, 821, 770]
S.ToList 704.2 ms -> [687, 702, 709, 691, 694, 710, 696, 698, 700, 694, 701, 719, 706, 694, 702, 699, 699, 703, 704, 701, 703, 705, 697, 707, 691, 697, 707, 692, 721, 698, 695, 700, 704, 700, 701, 710, 700, 705, 697, 711, 694, 700, 695, 698, 701, 692, 696, 702, 690, 699, 708, 700, 703, 714, 701, 697, 700, 699, 694, 701, 697, 696, 699, 694, 709, 1068, 690, 706, 699, 699, 695, 708, 695, 704, 704, 700, 695, 704, 695, 696, 702, 700, 710, 708, 693, 697, 702, 694, 700, 706, 699, 695, 706, 714, 704, 700, 695, 697, 707, 704]
S.ToArray 742.5 ms -> [742, 743, 733, 745, 741, 724, 738, 745, 728, 732, 740, 727, 739, 740, 726, 744, 758, 732, 744, 745, 730, 739, 738, 723, 745, 757, 729, 741, 736, 724, 744, 756, 739, 766, 737, 725, 741, 742, 736, 748, 742, 721, 746, 1043, 806, 747, 731, 727, 742, 742, 726, 738, 746, 727, 739, 743, 730, 744, 753, 741, 739, 746, 728, 740, 744, 734, 734, 738, 731, 747, 736, 731, 765, 735, 726, 740, 743, 730, 746, 742, 725, 731, 757, 734, 738, 741, 732, 747, 744, 721, 742, 741, 727, 745, 740, 730, 747, 760, 737, 740]
C1.ToList 32.34 ms -> [35, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31, 31, 31, 33, 32, 31, 31, 31, 31, 30, 32, 31, 31, 31, 31, 32, 30, 31, 31, 31, 30, 32, 31, 31, 31, 36, 31, 31, 31, 32, 30, 31, 32, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 33, 32, 31, 32, 31, 31, 33, 31, 31, 31, 31, 31, 32, 31, 32, 31, 34, 38, 68, 42, 79, 33, 31, 31, 31, 31, 31, 30, 30, 30, 30, 31, 31, 31, 31, 32, 31, 32, 31, 31, 31, 32, 33, 33, 31, 31]
C1.ToArray 56.32 ms -> [57, 56, 59, 54, 54, 55, 56, 57, 54, 54, 55, 55, 57, 56, 59, 57, 56, 58, 56, 56, 54, 56, 57, 55, 55, 55, 57, 58, 57, 58, 55, 55, 56, 55, 57, 56, 56, 59, 56, 56, 56, 56, 58, 56, 57, 56, 56, 57, 56, 55, 56, 56, 56, 59, 56, 56, 56, 55, 55, 54, 55, 54, 57, 56, 56, 56, 55, 55, 56, 56, 56, 59, 56, 56, 57, 56, 57, 56, 56, 56, 56, 62, 55, 56, 56, 56, 69, 57, 58, 56, 57, 58, 56, 57, 56, 56, 56, 56, 56, 56]
S1.ToList 88.69 ms -> [96, 90, 90, 89, 91, 88, 89, 90, 96, 89, 89, 89, 90, 90, 90, 89, 90, 90, 89, 90, 89, 91, 89, 91, 89, 91, 89, 90, 90, 89, 87, 88, 87, 88, 87, 87, 87, 87, 88, 88, 87, 87, 89, 87, 87, 87, 91, 88, 87, 86, 89, 87, 90, 89, 89, 90, 89, 87, 87, 87, 86, 87, 88, 90, 88, 87, 87, 92, 87, 87, 88, 88, 88, 86, 86, 87, 88, 87, 87, 87, 89, 87, 89, 87, 90, 89, 89, 89, 91, 89, 90, 89, 90, 88, 90, 90, 90, 88, 89, 89]
S1.ToArray 143.26 ms -> [130, 129, 130, 131, 133, 130, 131, 130, 135, 137, 130, 136, 132, 131, 130, 131, 132, 130, 132, 136, 130, 131, 157, 153, 194, 364, 176, 189, 203, 194, 189, 192, 183, 140, 142, 147, 145, 134, 159, 158, 142, 167, 130, 143, 145, 144, 160, 154, 156, 153, 153, 164, 142, 145, 137, 134, 145, 143, 142, 135, 133, 133, 135, 134, 134, 139, 139, 133, 134, 141, 133, 132, 133, 132, 133, 131, 135, 132, 133, 132, 128, 128, 130, 132, 129, 129, 129, 129, 129, 128, 134, 129, 129, 129, 129, 128, 128, 137, 130, 131]
C2.ToList 3.25 ms -> [5, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3]
C2.ToArray 3.37 ms -> [4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 5, 4, 9, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 4, 3, 3, 4, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 4, 4, 3, 4, 4, 3, 3, 4, 3, 3, 4, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 3, 3, 4, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3]
S2.ToList 37.72 ms -> [38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 40, 38, 38, 39, 39, 38, 38, 38, 38, 37, 37, 37, 37, 39, 37, 37, 39, 38, 37, 37, 37, 37, 39, 38, 37, 37, 38, 37, 38, 37, 37, 38, 37, 37, 37, 38, 37, 37, 36, 37, 38, 37, 39, 37, 39, 38, 37, 38, 38, 38, 38, 38, 38, 37, 38, 38, 38, 38, 38, 37, 38, 37, 37, 38, 37, 37, 39, 41, 37, 38, 38, 37, 37, 37, 37, 38, 37, 37, 37, 40, 37, 37, 37, 37, 39, 38]
S2.ToArray 38.86 ms -> [39, 37, 39, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 39, 38, 38, 38, 38, 38, 38, 38, 38, 39, 39, 38, 38, 38, 39, 37, 38, 38, 38, 38, 38, 37, 37, 38, 37, 37, 38, 38, 40, 38, 38, 38, 38, 38, 39, 38, 38, 39, 38, 38, 39, 38, 38, 40, 38, 39, 38, 38, 39, 38, 38, 38, 38, 38, 39, 38, 38, 38, 39, 39, 37, 38, 38, 39, 71, 78, 37, 37, 37, 39, 38, 38, 39, 38, 38, 38, 38, 38, 39, 38, 38, 38, 39, 38, 38, 38]
Group2上:
C.ToList 756.81 ms
C.ToArray 774.21 ms
S.ToList 709.7 ms
S.ToArray 753.51 ms
C1.ToList 32.06 ms
C1.ToArray 56.58 ms
S1.ToList 89.43 ms
S1.ToArray 132.85 ms
C2.ToList 3.45 ms
C2.ToArray 3.36 ms
S2.ToList 41.43 ms
S2.ToArray 40.84 ms
Group3:
C.ToList 756.64 ms
C.ToArray 771.56 ms
S.ToList 705.42 ms
S.ToArray 749.59 ms
C1.ToList 31.45 ms
C1.ToArray 57.03 ms
S1.ToList 91.26 ms
S1.ToArray 129.77 ms
C2.ToList 3.26 ms
C2.ToArray 3.29 ms
S2.ToList 41.57 ms
S2.ToArray 40.69 ms
Group4:
C.ToList 729.65 ms -> [749, 730, 721, 719, 723, 743, 721, 724, 727, 722, 716, 725, 723, 726, 718, 722, 731, 722, 723, 725, 723, 722, 728, 726, 728, 718, 726, 1088, 788, 737, 729, 710, 730, 728, 717, 723, 728, 721, 722, 728, 722, 736, 723, 729, 732, 724, 726, 727, 728, 728, 726, 726, 725, 727, 725, 728, 728, 718, 724, 725, 726, 724, 726, 729, 727, 722, 722, 725, 725, 728, 724, 727, 738, 717, 726, 723, 725, 725, 727, 724, 720, 726, 726, 723, 727, 730, 723, 721, 725, 727, 727, 733, 720, 722, 722, 725, 722, 725, 728, 726]
C.ToArray 788.36 ms -> [748, 740, 742, 797, 1090, 774, 781, 787, 784, 786, 786, 782, 781, 781, 784, 783, 783, 781, 783, 787, 783, 784, 775, 789, 784, 785, 778, 774, 781, 783, 786, 781, 780, 788, 778, 785, 777, 781, 786, 782, 781, 787, 782, 787, 784, 773, 783, 782, 781, 777, 783, 781, 785, 788, 777, 776, 784, 784, 783, 789, 778, 781, 791, 768, 779, 783, 781, 787, 786, 781, 784, 781, 785, 781, 780, 809, 1155, 780, 790, 789, 783, 776, 785, 783, 786, 787, 782, 782, 787, 777, 779, 784, 783, 776, 786, 775, 782, 779, 784, 784]
S.ToList 705.54 ms -> [690, 705, 709, 708, 702, 707, 703, 696, 703, 702, 700, 703, 700, 707, 705, 699, 697, 703, 695, 698, 707, 697, 711, 710, 699, 700, 708, 707, 693, 710, 704, 691, 702, 700, 703, 700, 705, 700, 703, 695, 709, 705, 698, 699, 709, 700, 699, 704, 691, 705, 703, 700, 708, 1048, 710, 706, 706, 692, 702, 705, 695, 701, 710, 697, 698, 706, 705, 707, 707, 695, 698, 704, 698, 699, 705, 698, 703, 702, 701, 697, 702, 702, 704, 703, 699, 707, 703, 705, 701, 717, 698, 695, 713, 696, 708, 705, 697, 699, 700, 698]
S.ToArray 745.01 ms -> [751, 743, 727, 734, 736, 745, 739, 750, 739, 750, 758, 739, 744, 738, 730, 744, 745, 739, 744, 750, 733, 735, 743, 731, 749, 748, 727, 746, 749, 731, 737, 803, 1059, 756, 769, 748, 740, 745, 741, 746, 749, 732, 741, 742, 732, 744, 746, 737, 742, 739, 733, 744, 741, 729, 746, 760, 725, 741, 764, 739, 750, 751, 727, 745, 738, 727, 735, 741, 720, 736, 740, 733, 741, 746, 731, 749, 756, 740, 738, 736, 732, 741, 741, 733, 741, 744, 736, 742, 742, 735, 743, 746, 729, 748, 765, 743, 734, 742, 728, 749]
C1.ToList 32.27 ms -> [36, 31, 31, 32, 31, 32, 31, 30, 32, 30, 30, 30, 34, 32, 31, 31, 31, 31, 31, 31, 31, 32, 38, 51, 68, 57, 35, 30, 31, 31, 30, 30, 33, 30, 31, 34, 31, 34, 32, 31, 31, 31, 31, 32, 30, 30, 31, 30, 31, 31, 32, 31, 31, 31, 32, 31, 31, 31, 32, 31, 33, 31, 31, 32, 30, 30, 30, 30, 30, 33, 30, 33, 32, 31, 30, 31, 31, 32, 32, 31, 35, 31, 34, 31, 31, 32, 31, 31, 32, 31, 32, 31, 31, 35, 31, 31, 31, 31, 31, 32]
C1.ToArray 56.72 ms -> [58, 56, 57, 57, 59, 58, 58, 57, 56, 59, 57, 55, 55, 54, 56, 55, 56, 56, 57, 59, 56, 55, 58, 56, 55, 55, 55, 55, 58, 58, 55, 57, 57, 56, 57, 57, 57, 57, 59, 59, 56, 57, 56, 57, 57, 56, 57, 59, 58, 56, 57, 57, 57, 58, 56, 56, 59, 56, 59, 57, 57, 57, 57, 59, 57, 56, 57, 56, 58, 56, 57, 56, 57, 59, 55, 58, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 56, 56, 57, 56, 56, 57, 58, 57, 57, 57, 57, 57]
S1.ToList 90.72 ms -> [95, 90, 90, 89, 89, 89, 91, 89, 89, 87, 91, 89, 89, 89, 91, 89, 89, 89, 90, 89, 89, 90, 88, 89, 88, 90, 89, 90, 89, 89, 90, 90, 89, 89, 90, 91, 89, 91, 89, 90, 89, 89, 90, 91, 89, 89, 89, 89, 89, 89, 90, 89, 89, 89, 90, 89, 90, 89, 91, 89, 90, 89, 90, 89, 90, 89, 96, 89, 90, 89, 89, 89, 89, 89, 90, 89, 89, 89, 90, 87, 89, 90, 90, 91, 89, 91, 89, 89, 90, 91, 90, 89, 93, 144, 149, 90, 90, 89, 89, 89]
S1.ToArray 131.4 ms -> [130, 128, 127, 134, 129, 129, 130, 136, 131, 130, 132, 132, 133, 131, 132, 131, 133, 132, 130, 131, 132, 131, 130, 133, 133, 130, 130, 131, 131, 131, 132, 134, 131, 131, 132, 131, 132, 131, 134, 131, 131, 130, 131, 131, 130, 132, 129, 131, 131, 131, 132, 131, 133, 134, 131, 131, 132, 132, 131, 133, 131, 131, 130, 133, 131, 130, 134, 132, 131, 132, 132, 131, 131, 134, 131, 131, 132, 132, 131, 130, 138, 130, 130, 131, 132, 132, 130, 134, 131, 131, 132, 131, 130, 132, 133, 131, 131, 131, 130, 131]
C2.ToList 3.21 ms -> [4, 3, 3, 3, 4, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 3, 3, 3]
C2.ToArray 3.22 ms -> [4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 5, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 4]
S2.ToList 41.46 ms -> [42, 40, 41, 40, 42, 40, 40, 40, 40, 40, 40, 40, 40, 41, 40, 40, 41, 40, 40, 40, 39, 41, 41, 39, 40, 40, 43, 40, 39, 40, 40, 40, 40, 40, 40, 41, 40, 40, 40, 43, 40, 43, 75, 76, 47, 39, 40, 40, 40, 40, 42, 40, 41, 40, 40, 40, 44, 41, 40, 42, 42, 40, 41, 41, 41, 41, 41, 40, 41, 41, 41, 41, 42, 41, 40, 41, 41, 42, 42, 41, 40, 41, 41, 41, 41, 41, 40, 42, 40, 42, 41, 41, 41, 43, 41, 41, 41, 41, 42, 41]
S2.ToArray 41.14 ms -> [42, 41, 41, 40, 40, 40, 40, 41, 41, 42, 41, 42, 41, 41, 41, 42, 41, 41, 42, 41, 41, 41, 41, 41, 42, 40, 41, 40, 42, 40, 42, 41, 40, 42, 41, 41, 43, 42, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 40, 40, 41, 41, 41, 40, 42, 41, 41, 41, 41, 41, 40, 41, 41, 42, 41, 41, 41, 42, 41, 41, 41, 41, 41, 41, 42, 42, 42, 41, 45, 46, 41, 40, 41, 41, 42, 41, 41, 41, 41, 41, 41, 40, 41, 43, 40, 40, 40, 40, 43, 41]
Group5:
C.ToList 757.06 ms -> [770, 752, 752, 751, 778, 763, 761, 763, 747, 758, 748, 747, 754, 749, 752, 753, 756, 762, 750, 753, 756, 749, 755, 757, 755, 756, 755, 744, 753, 758, 747, 751, 759, 751, 761, 755, 746, 752, 752, 749, 746, 752, 753, 755, 752, 755, 754, 754, 966, 937, 749, 759, 748, 747, 754, 749, 755, 750, 746, 754, 757, 752, 753, 745, 758, 755, 761, 753, 751, 755, 755, 752, 746, 756, 755, 746, 742, 751, 751, 749, 752, 751, 756, 756, 755, 742, 749, 754, 749, 756, 753, 751, 754, 752, 751, 754, 753, 749, 755, 756]
C.ToArray 772.8 ms -> [766, 772, 755, 763, 758, 767, 763, 762, 761, 768, 769, 763, 770, 757, 765, 760, 766, 759, 764, 761, 760, 777, 1102, 881, 759, 765, 758, 762, 772, 761, 758, 757, 765, 769, 769, 761, 762, 762, 763, 760, 770, 764, 760, 768, 758, 766, 763, 770, 769, 761, 764, 761, 761, 767, 761, 762, 764, 757, 765, 766, 767, 771, 753, 762, 769, 768, 759, 764, 764, 760, 763, 763, 763, 763, 763, 767, 761, 771, 760, 765, 760, 758, 768, 770, 751, 771, 767, 771, 765, 763, 760, 765, 765, 769, 767, 767, 1193, 774, 767, 764]
S.ToList 704.73 ms -> [682, 708, 705, 699, 705, 704, 695, 703, 702, 699, 701, 708, 699, 702, 703, 701, 701, 699, 701, 707, 707, 700, 701, 705, 700, 697, 706, 702, 701, 706, 699, 692, 702, 697, 707, 704, 697, 698, 699, 699, 702, 703, 698, 697, 702, 703, 702, 704, 694, 697, 707, 695, 711, 710, 700, 693, 703, 699, 699, 706, 698, 701, 703, 704, 698, 706, 700, 704, 701, 699, 702, 705, 694, 698, 709, 736, 1053, 704, 694, 700, 698, 696, 701, 700, 700, 706, 706, 692, 698, 707, 703, 695, 703, 699, 694, 708, 695, 694, 706, 695]
S.ToArray 744.17 ms -> [746, 740, 725, 740, 739, 731, 746, 760, 735, 738, 740, 734, 744, 748, 737, 744, 745, 727, 736, 738, 728, 743, 745, 735, 748, 760, 739, 748, 762, 742, 741, 747, 733, 746, 758, 742, 742, 741, 724, 744, 747, 727, 740, 740, 729, 742, 757, 741, 740, 742, 726, 739, 746, 1133, 749, 737, 730, 740, 747, 733, 747, 752, 731, 747, 742, 730, 741, 749, 731, 749, 743, 730, 747, 742, 731, 737, 745, 734, 739, 735, 727, 743, 752, 731, 744, 742, 729, 740, 746, 731, 739, 746, 733, 745, 743, 733, 739, 742, 727, 737]
C1.ToList 31.71 ms -> [35, 32, 32, 30, 31, 33, 31, 32, 32, 31, 31, 32, 32, 33, 32, 31, 31, 32, 31, 32, 32, 32, 31, 32, 33, 32, 31, 31, 31, 32, 31, 34, 31, 31, 32, 33, 32, 32, 31, 32, 34, 32, 31, 32, 33, 31, 32, 32, 31, 32, 32, 32, 32, 32, 32, 31, 31, 32, 31, 33, 30, 31, 32, 30, 30, 33, 32, 32, 34, 31, 31, 31, 31, 32, 31, 31, 31, 31, 32, 31, 31, 33, 31, 32, 32, 32, 33, 32, 31, 31, 31, 31, 31, 32, 32, 33, 32, 31, 31, 32]
C1.ToArray 59.53 ms -> [63, 57, 58, 58, 57, 59, 59, 57, 60, 131, 127, 67, 58, 56, 59, 56, 57, 58, 58, 58, 57, 59, 60, 57, 57, 59, 60, 57, 57, 57, 58, 58, 58, 58, 57, 57, 61, 57, 58, 57, 57, 57, 57, 57, 58, 58, 58, 58, 57, 58, 59, 57, 58, 57, 57, 59, 58, 58, 59, 57, 59, 57, 56, 56, 59, 56, 56, 59, 57, 58, 58, 58, 57, 58, 59, 59, 58, 57, 58, 62, 65, 57, 57, 57, 58, 60, 59, 58, 59, 57, 58, 57, 58, 59, 58, 58, 58, 59, 60, 58]
S1.ToList 82.78 ms -> [87, 82, 83, 83, 82, 82, 83, 84, 82, 83, 84, 84, 84, 82, 82, 84, 82, 84, 83, 84, 82, 82, 82, 81, 83, 83, 83, 84, 84, 82, 82, 83, 83, 83, 82, 83, 85, 83, 82, 82, 84, 82, 82, 83, 83, 83, 82, 82, 82, 83, 82, 83, 82, 84, 82, 83, 82, 83, 82, 82, 82, 84, 82, 83, 82, 82, 86, 83, 83, 82, 83, 83, 83, 82, 84, 82, 83, 81, 82, 82, 82, 82, 83, 83, 83, 82, 83, 84, 83, 82, 83, 83, 83, 82, 83, 84, 82, 82, 83, 83]
S1.ToArray 122.3 ms -> [122, 119, 119, 120, 119, 120, 120, 121, 119, 119, 122, 120, 120, 120, 122, 120, 123, 120, 120, 120, 121, 123, 120, 120, 120, 121, 120, 121, 122, 120, 123, 119, 121, 118, 121, 120, 120, 120, 119, 124, 119, 121, 119, 120, 120, 120, 120, 120, 122, 121, 123, 230, 203, 123, 119, 119, 122, 119, 120, 120, 120, 122, 120, 121, 120, 121, 120, 121, 120, 121, 120, 120, 120, 121, 122, 121, 123, 119, 119, 119, 119, 121, 120, 120, 120, 122, 121, 122, 119, 120, 120, 121, 121, 120, 121, 120, 121, 118, 118, 118]
C2.ToList 3.43 ms -> [5, 3, 4, 4, 4, 3, 4, 4, 4, 4, 4, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 4, 3, 4, 4, 3, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 6, 4, 4, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 4, 4, 3, 4, 4, 3, 4, 4, 4, 4, 4, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3]
C2.ToArray 3.48 ms -> [3, 3, 3, 3, 4, 4, 3, 4, 4, 4, 3, 4, 3, 3, 4, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 3, 3, 4, 3, 3, 4, 4, 3, 3, 4, 3, 4, 4, 3, 4, 4, 4, 4, 4, 3, 3, 3, 4, 4, 3, 4, 4, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 3, 4, 4, 4, 4, 3]
S2.ToList 41.47 ms -> [41, 41, 49, 67, 82, 41, 41, 40, 40, 40, 40, 40, 41, 40, 40, 40, 40, 40, 41, 40, 42, 42, 40, 40, 41, 41, 41, 40, 41, 40, 41, 40, 41, 40, 42, 41, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 41, 41, 41, 41, 41, 42, 41, 41, 41, 42, 40, 41, 40, 40, 40, 42, 40, 41, 42, 41, 42, 41, 42, 40, 41, 41, 41, 41, 41, 41, 41, 41, 40, 41, 40, 41, 41, 41, 40, 41, 41, 40, 40, 41, 41, 41, 41, 41, 43, 40, 40, 41, 42, 41]
S2.ToArray 40.62 ms -> [42, 41, 44, 40, 40, 40, 40, 41, 41, 40, 41, 41, 41, 40, 41, 41, 40, 41, 41, 40, 41, 40, 40, 41, 42, 41, 41, 41, 40, 40, 40, 40, 40, 41, 41, 42, 40, 41, 41, 41, 41, 41, 40, 42, 40, 40, 41, 41, 41, 40, 41, 40, 40, 40, 40, 40, 41, 40, 40, 41, 40, 40, 40, 40, 41, 40, 41, 41, 41, 40, 41, 41, 40, 41, 40, 41, 42, 40, 41, 41, 42, 41, 41, 40, 41, 40, 41, 40, 41, 41, 40, 40, 40, 41, 41, 40, 40, 40, 40, 40]
由于stackoverflow对应答的字符数有限制，因此省略了Group2和Group3的示例列表。
如您所见，在大多数情况下使用ToList或ToArry并不重要。
在处理运行时计算的IEnumerable<T>对象时，如果计算带来的负载比ToList和ToArray的内存分配和复制操作重，则差异不显著(C.ToList vs .ToArray, S.ToList vs .ToArray)。
The difference can be observed only on non-runtime-calculated IEnumerable<T> only objects (C1.ToList vs C1.ToArray and S1.ToList vs S1.ToArray). But the absolute difference (<60ms) is still acceptable on an one million small object IEnumerable<T>. In fact, the difference is decided by the implementation of Enumerator<T> of IEnumerable<T>. So, if your program is really really really sensitive on this, you do have to profile, profile, profile! At last you'll probably find that the bottleneck is not on ToList or ToArray, but the detail of enumerators.
C2的结果。ToList vs C2。ToArray和S2。ToList vs S2。ToArray表明，您真的不需要关心非运行时计算的ICollection<T>对象上的ToList或ToArray。
当然，这只是在我的机器上的结果，这些操作在不同机器上的实际花费时间是不一样的，你可以在你的机器上使用上面的代码来发现。
你需要做出选择的唯一原因是，你在List<T>或T[]上有特定的需求，正如@Jeppe Stig Nielsen的回答所描述的那样。
其他回答
性能差异并不显著，因为List<T>是作为动态大小的数组实现的。调用ToArray()(它使用内部Buffer<T>类来增长数组)或ToList()(它调用List<T>(IEnumerable<T>)构造函数)将最终成为将它们放入数组并增长数组直到适合它们为止的问题。
如果您希望具体确认这一事实，请查看Reflector中所讨论的方法的实现——您将看到它们的代码几乎完全相同。
我知道这是一个老帖子，但在有了同样的问题和做了一些研究之后，我发现了一些有趣的东西，可能值得分享。
首先，我同意@mquander和他的回答。在性能方面，两者是相同的。
但是，我一直在使用Reflector查看System.Linq.Enumerable扩展名称空间中的方法，并注意到一个非常常见的优化。 只要可能，IEnumerable<T>源就转换为IList<T>或ICollection<T>来优化方法。例如，查看ElementAt(int)。
有趣的是，微软选择只优化IList<T>，而不是IList。微软似乎更喜欢使用IList<T>接口。
老问题，但总是有新的提问者。
根据来源的System.Linq。可枚举，ToList只返回一个新的List(source)，而ToArray使用一个新的Buffer<T>(source).ToArray()返回一个T[]。
关于内存分配:
当运行在IEnumerable<T> only对象上时，ToArray确实比ToList多分配一次内存。但是在大多数情况下您不必关心它，因为GC将在需要时进行垃圾收集。
关于运行时效率:
那些质疑这个问题的人可以在自己的机器上运行下面的代码，然后就会得到答案。
class PersonC
{
public Guid uuid;
public string name;
public int age;
public bool sex;
public DateTime BirthDay;
public double weight;
}
struct PersonS
{
public Guid uuid;
public string name;
public int age;
public bool sex;
public DateTime BirthDay;
public double weight;
}
class PersonT<T> : IEnumerable<T>
{
private List<T> items;
public PersonT(IEnumerable<T> init)
{
items = new List<T>(init);
}
public IEnumerator<T> GetEnumerator() => items.GetEnumerator();
IEnumerator IEnumerable.GetEnumerator() => items.GetEnumerator();
}
private IEnumerable<PersonC> C(int count)
{
for (var i = 0; i < count; ++i)
{
var guid = Guid.NewGuid();
var guidBytes = guid.ToByteArray(); //16 bytes
yield return new PersonC
{
uuid = guid,
name = guid.ToString(),
age = guidBytes[0] ^ guidBytes[7],
sex = guidBytes[14] % 2 == 0,
BirthDay = DateTime.Now.AddDays(-guidBytes[11] * 18),
weight = guidBytes[12] * 100
};
}
}
private IEnumerable<PersonS> S(int count)
{
for (var i = 0; i < count; ++i)
{
var guid = Guid.NewGuid();
var guidBytes = guid.ToByteArray(); //16 bytes
yield return new PersonS
{
uuid = guid,
name = guid.ToString(),
age = guidBytes[0] ^ guidBytes[7],
sex = guidBytes[14] % 2 == 0,
BirthDay = DateTime.Now.AddDays(-guidBytes[11] * 18),
weight = guidBytes[12] * 100
};
}
}
private void MakeLog(string test, List<long> log) =>
Console.WriteLine("{0} {1} ms -> [{2}]",
test,
log.Average(),
string.Join(", ", log)
);
private void Test1(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
MakeLog("C.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = C(count).ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = C(count).ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = S(count).ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = S(count).ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void Test2(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
var dataC1 = new PersonT<PersonC>(C(count));
var dataS1 = new PersonT<PersonS>(S(count));
MakeLog("C1.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC1.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C1.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC1.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S1.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS1.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S1.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS1.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void Test3(int times, int count)
{
var test = Enumerable.Range(1, times).ToArray();
var dataC2 = (ICollection<PersonC>) new List<PersonC>(C(count));
var dataS2 = (ICollection<PersonS>) new List<PersonS>(S(count));
MakeLog("C2.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC2.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("C2.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataC2.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S2.ToList", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS2.ToList();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
MakeLog("S2.ToArray", test.Select(o =>
{
var sw = new Stopwatch();
GC.Collect();
sw.Start();
var ret = dataS2.ToArray();
sw.Stop();
return sw.ElapsedMilliseconds;
}).ToList());
}
private void TestMain()
{
const int times = 100;
const int count = 1_000_000 + 1;
Test1(times, count);
Test2(times, count);
Test3(times, count);
}
我在我的机器上得到了这些结果:
Group1:
C.ToList 761.79 ms -> [775, 755, 759, 759, 756, 759, 765, 750, 757, 762, 759, 754, 757, 753, 763, 753, 759, 756, 768, 754, 763, 757, 757, 777, 780, 758, 754, 758, 762, 754, 758, 757, 763, 758, 760, 754, 761, 755, 764, 847, 952, 755, 747, 763, 760, 758, 754, 763, 761, 758, 750, 764, 757, 763, 762, 756, 753, 759, 759, 757, 758, 779, 765, 760, 760, 756, 760, 756, 755, 764, 759, 753, 757, 760, 752, 764, 758, 760, 758, 760, 755, 761, 751, 753, 761, 762, 761, 758, 759, 752, 765, 756, 760, 755, 757, 753, 760, 751, 755, 779]
C.ToArray 782.56 ms -> [783, 774, 771, 771, 773, 774, 775, 775, 772, 770, 771, 774, 771, 1023, 975, 772, 767, 776, 771, 779, 772, 779, 775, 771, 775, 773, 775, 771, 765, 774, 770, 781, 772, 771, 781, 762, 817, 770, 775, 779, 769, 774, 763, 775, 777, 769, 777, 772, 775, 778, 775, 771, 770, 774, 772, 769, 772, 769, 774, 775, 768, 775, 769, 774, 771, 776, 774, 773, 778, 769, 778, 767, 770, 787, 783, 779, 771, 768, 805, 780, 779, 767, 773, 771, 773, 785, 1044, 853, 775, 774, 775, 771, 770, 769, 770, 776, 770, 780, 821, 770]
S.ToList 704.2 ms -> [687, 702, 709, 691, 694, 710, 696, 698, 700, 694, 701, 719, 706, 694, 702, 699, 699, 703, 704, 701, 703, 705, 697, 707, 691, 697, 707, 692, 721, 698, 695, 700, 704, 700, 701, 710, 700, 705, 697, 711, 694, 700, 695, 698, 701, 692, 696, 702, 690, 699, 708, 700, 703, 714, 701, 697, 700, 699, 694, 701, 697, 696, 699, 694, 709, 1068, 690, 706, 699, 699, 695, 708, 695, 704, 704, 700, 695, 704, 695, 696, 702, 700, 710, 708, 693, 697, 702, 694, 700, 706, 699, 695, 706, 714, 704, 700, 695, 697, 707, 704]
S.ToArray 742.5 ms -> [742, 743, 733, 745, 741, 724, 738, 745, 728, 732, 740, 727, 739, 740, 726, 744, 758, 732, 744, 745, 730, 739, 738, 723, 745, 757, 729, 741, 736, 724, 744, 756, 739, 766, 737, 725, 741, 742, 736, 748, 742, 721, 746, 1043, 806, 747, 731, 727, 742, 742, 726, 738, 746, 727, 739, 743, 730, 744, 753, 741, 739, 746, 728, 740, 744, 734, 734, 738, 731, 747, 736, 731, 765, 735, 726, 740, 743, 730, 746, 742, 725, 731, 757, 734, 738, 741, 732, 747, 744, 721, 742, 741, 727, 745, 740, 730, 747, 760, 737, 740]
C1.ToList 32.34 ms -> [35, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31, 31, 31, 33, 32, 31, 31, 31, 31, 30, 32, 31, 31, 31, 31, 32, 30, 31, 31, 31, 30, 32, 31, 31, 31, 36, 31, 31, 31, 32, 30, 31, 32, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 33, 32, 31, 32, 31, 31, 33, 31, 31, 31, 31, 31, 32, 31, 32, 31, 34, 38, 68, 42, 79, 33, 31, 31, 31, 31, 31, 30, 30, 30, 30, 31, 31, 31, 31, 32, 31, 32, 31, 31, 31, 32, 33, 33, 31, 31]
C1.ToArray 56.32 ms -> [57, 56, 59, 54, 54, 55, 56, 57, 54, 54, 55, 55, 57, 56, 59, 57, 56, 58, 56, 56, 54, 56, 57, 55, 55, 55, 57, 58, 57, 58, 55, 55, 56, 55, 57, 56, 56, 59, 56, 56, 56, 56, 58, 56, 57, 56, 56, 57, 56, 55, 56, 56, 56, 59, 56, 56, 56, 55, 55, 54, 55, 54, 57, 56, 56, 56, 55, 55, 56, 56, 56, 59, 56, 56, 57, 56, 57, 56, 56, 56, 56, 62, 55, 56, 56, 56, 69, 57, 58, 56, 57, 58, 56, 57, 56, 56, 56, 56, 56, 56]
S1.ToList 88.69 ms -> [96, 90, 90, 89, 91, 88, 89, 90, 96, 89, 89, 89, 90, 90, 90, 89, 90, 90, 89, 90, 89, 91, 89, 91, 89, 91, 89, 90, 90, 89, 87, 88, 87, 88, 87, 87, 87, 87, 88, 88, 87, 87, 89, 87, 87, 87, 91, 88, 87, 86, 89, 87, 90, 89, 89, 90, 89, 87, 87, 87, 86, 87, 88, 90, 88, 87, 87, 92, 87, 87, 88, 88, 88, 86, 86, 87, 88, 87, 87, 87, 89, 87, 89, 87, 90, 89, 89, 89, 91, 89, 90, 89, 90, 88, 90, 90, 90, 88, 89, 89]
S1.ToArray 143.26 ms -> [130, 129, 130, 131, 133, 130, 131, 130, 135, 137, 130, 136, 132, 131, 130, 131, 132, 130, 132, 136, 130, 131, 157, 153, 194, 364, 176, 189, 203, 194, 189, 192, 183, 140, 142, 147, 145, 134, 159, 158, 142, 167, 130, 143, 145, 144, 160, 154, 156, 153, 153, 164, 142, 145, 137, 134, 145, 143, 142, 135, 133, 133, 135, 134, 134, 139, 139, 133, 134, 141, 133, 132, 133, 132, 133, 131, 135, 132, 133, 132, 128, 128, 130, 132, 129, 129, 129, 129, 129, 128, 134, 129, 129, 129, 129, 128, 128, 137, 130, 131]
C2.ToList 3.25 ms -> [5, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3]
C2.ToArray 3.37 ms -> [4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 5, 4, 9, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 4, 3, 3, 4, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 4, 4, 3, 4, 4, 3, 3, 4, 3, 3, 4, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 3, 3, 4, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3]
S2.ToList 37.72 ms -> [38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 40, 38, 38, 39, 39, 38, 38, 38, 38, 37, 37, 37, 37, 39, 37, 37, 39, 38, 37, 37, 37, 37, 39, 38, 37, 37, 38, 37, 38, 37, 37, 38, 37, 37, 37, 38, 37, 37, 36, 37, 38, 37, 39, 37, 39, 38, 37, 38, 38, 38, 38, 38, 38, 37, 38, 38, 38, 38, 38, 37, 38, 37, 37, 38, 37, 37, 39, 41, 37, 38, 38, 37, 37, 37, 37, 38, 37, 37, 37, 40, 37, 37, 37, 37, 39, 38]
S2.ToArray 38.86 ms -> [39, 37, 39, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 39, 38, 38, 38, 38, 38, 38, 38, 38, 39, 39, 38, 38, 38, 39, 37, 38, 38, 38, 38, 38, 37, 37, 38, 37, 37, 38, 38, 40, 38, 38, 38, 38, 38, 39, 38, 38, 39, 38, 38, 39, 38, 38, 40, 38, 39, 38, 38, 39, 38, 38, 38, 38, 38, 39, 38, 38, 38, 39, 39, 37, 38, 38, 39, 71, 78, 37, 37, 37, 39, 38, 38, 39, 38, 38, 38, 38, 38, 39, 38, 38, 38, 39, 38, 38, 38]
Group2上:
C.ToList 756.81 ms
C.ToArray 774.21 ms
S.ToList 709.7 ms
S.ToArray 753.51 ms
C1.ToList 32.06 ms
C1.ToArray 56.58 ms
S1.ToList 89.43 ms
S1.ToArray 132.85 ms
C2.ToList 3.45 ms
C2.ToArray 3.36 ms
S2.ToList 41.43 ms
S2.ToArray 40.84 ms
Group3:
C.ToList 756.64 ms
C.ToArray 771.56 ms
S.ToList 705.42 ms
S.ToArray 749.59 ms
C1.ToList 31.45 ms
C1.ToArray 57.03 ms
S1.ToList 91.26 ms
S1.ToArray 129.77 ms
C2.ToList 3.26 ms
C2.ToArray 3.29 ms
S2.ToList 41.57 ms
S2.ToArray 40.69 ms
Group4:
C.ToList 729.65 ms -> [749, 730, 721, 719, 723, 743, 721, 724, 727, 722, 716, 725, 723, 726, 718, 722, 731, 722, 723, 725, 723, 722, 728, 726, 728, 718, 726, 1088, 788, 737, 729, 710, 730, 728, 717, 723, 728, 721, 722, 728, 722, 736, 723, 729, 732, 724, 726, 727, 728, 728, 726, 726, 725, 727, 725, 728, 728, 718, 724, 725, 726, 724, 726, 729, 727, 722, 722, 725, 725, 728, 724, 727, 738, 717, 726, 723, 725, 725, 727, 724, 720, 726, 726, 723, 727, 730, 723, 721, 725, 727, 727, 733, 720, 722, 722, 725, 722, 725, 728, 726]
C.ToArray 788.36 ms -> [748, 740, 742, 797, 1090, 774, 781, 787, 784, 786, 786, 782, 781, 781, 784, 783, 783, 781, 783, 787, 783, 784, 775, 789, 784, 785, 778, 774, 781, 783, 786, 781, 780, 788, 778, 785, 777, 781, 786, 782, 781, 787, 782, 787, 784, 773, 783, 782, 781, 777, 783, 781, 785, 788, 777, 776, 784, 784, 783, 789, 778, 781, 791, 768, 779, 783, 781, 787, 786, 781, 784, 781, 785, 781, 780, 809, 1155, 780, 790, 789, 783, 776, 785, 783, 786, 787, 782, 782, 787, 777, 779, 784, 783, 776, 786, 775, 782, 779, 784, 784]
S.ToList 705.54 ms -> [690, 705, 709, 708, 702, 707, 703, 696, 703, 702, 700, 703, 700, 707, 705, 699, 697, 703, 695, 698, 707, 697, 711, 710, 699, 700, 708, 707, 693, 710, 704, 691, 702, 700, 703, 700, 705, 700, 703, 695, 709, 705, 698, 699, 709, 700, 699, 704, 691, 705, 703, 700, 708, 1048, 710, 706, 706, 692, 702, 705, 695, 701, 710, 697, 698, 706, 705, 707, 707, 695, 698, 704, 698, 699, 705, 698, 703, 702, 701, 697, 702, 702, 704, 703, 699, 707, 703, 705, 701, 717, 698, 695, 713, 696, 708, 705, 697, 699, 700, 698]
S.ToArray 745.01 ms -> [751, 743, 727, 734, 736, 745, 739, 750, 739, 750, 758, 739, 744, 738, 730, 744, 745, 739, 744, 750, 733, 735, 743, 731, 749, 748, 727, 746, 749, 731, 737, 803, 1059, 756, 769, 748, 740, 745, 741, 746, 749, 732, 741, 742, 732, 744, 746, 737, 742, 739, 733, 744, 741, 729, 746, 760, 725, 741, 764, 739, 750, 751, 727, 745, 738, 727, 735, 741, 720, 736, 740, 733, 741, 746, 731, 749, 756, 740, 738, 736, 732, 741, 741, 733, 741, 744, 736, 742, 742, 735, 743, 746, 729, 748, 765, 743, 734, 742, 728, 749]
C1.ToList 32.27 ms -> [36, 31, 31, 32, 31, 32, 31, 30, 32, 30, 30, 30, 34, 32, 31, 31, 31, 31, 31, 31, 31, 32, 38, 51, 68, 57, 35, 30, 31, 31, 30, 30, 33, 30, 31, 34, 31, 34, 32, 31, 31, 31, 31, 32, 30, 30, 31, 30, 31, 31, 32, 31, 31, 31, 32, 31, 31, 31, 32, 31, 33, 31, 31, 32, 30, 30, 30, 30, 30, 33, 30, 33, 32, 31, 30, 31, 31, 32, 32, 31, 35, 31, 34, 31, 31, 32, 31, 31, 32, 31, 32, 31, 31, 35, 31, 31, 31, 31, 31, 32]
C1.ToArray 56.72 ms -> [58, 56, 57, 57, 59, 58, 58, 57, 56, 59, 57, 55, 55, 54, 56, 55, 56, 56, 57, 59, 56, 55, 58, 56, 55, 55, 55, 55, 58, 58, 55, 57, 57, 56, 57, 57, 57, 57, 59, 59, 56, 57, 56, 57, 57, 56, 57, 59, 58, 56, 57, 57, 57, 58, 56, 56, 59, 56, 59, 57, 57, 57, 57, 59, 57, 56, 57, 56, 58, 56, 57, 56, 57, 59, 55, 58, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 56, 56, 57, 56, 56, 57, 58, 57, 57, 57, 57, 57]
S1.ToList 90.72 ms -> [95, 90, 90, 89, 89, 89, 91, 89, 89, 87, 91, 89, 89, 89, 91, 89, 89, 89, 90, 89, 89, 90, 88, 89, 88, 90, 89, 90, 89, 89, 90, 90, 89, 89, 90, 91, 89, 91, 89, 90, 89, 89, 90, 91, 89, 89, 89, 89, 89, 89, 90, 89, 89, 89, 90, 89, 90, 89, 91, 89, 90, 89, 90, 89, 90, 89, 96, 89, 90, 89, 89, 89, 89, 89, 90, 89, 89, 89, 90, 87, 89, 90, 90, 91, 89, 91, 89, 89, 90, 91, 90, 89, 93, 144, 149, 90, 90, 89, 89, 89]
S1.ToArray 131.4 ms -> [130, 128, 127, 134, 129, 129, 130, 136, 131, 130, 132, 132, 133, 131, 132, 131, 133, 132, 130, 131, 132, 131, 130, 133, 133, 130, 130, 131, 131, 131, 132, 134, 131, 131, 132, 131, 132, 131, 134, 131, 131, 130, 131, 131, 130, 132, 129, 131, 131, 131, 132, 131, 133, 134, 131, 131, 132, 132, 131, 133, 131, 131, 130, 133, 131, 130, 134, 132, 131, 132, 132, 131, 131, 134, 131, 131, 132, 132, 131, 130, 138, 130, 130, 131, 132, 132, 130, 134, 131, 131, 132, 131, 130, 132, 133, 131, 131, 131, 130, 131]
C2.ToList 3.21 ms -> [4, 3, 3, 3, 4, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 4, 3, 3, 3]
C2.ToArray 3.22 ms -> [4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 5, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 3, 4]
S2.ToList 41.46 ms -> [42, 40, 41, 40, 42, 40, 40, 40, 40, 40, 40, 40, 40, 41, 40, 40, 41, 40, 40, 40, 39, 41, 41, 39, 40, 40, 43, 40, 39, 40, 40, 40, 40, 40, 40, 41, 40, 40, 40, 43, 40, 43, 75, 76, 47, 39, 40, 40, 40, 40, 42, 40, 41, 40, 40, 40, 44, 41, 40, 42, 42, 40, 41, 41, 41, 41, 41, 40, 41, 41, 41, 41, 42, 41, 40, 41, 41, 42, 42, 41, 40, 41, 41, 41, 41, 41, 40, 42, 40, 42, 41, 41, 41, 43, 41, 41, 41, 41, 42, 41]
S2.ToArray 41.14 ms -> [42, 41, 41, 40, 40, 40, 40, 41, 41, 42, 41, 42, 41, 41, 41, 42, 41, 41, 42, 41, 41, 41, 41, 41, 42, 40, 41, 40, 42, 40, 42, 41, 40, 42, 41, 41, 43, 42, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 40, 40, 41, 41, 41, 40, 42, 41, 41, 41, 41, 41, 40, 41, 41, 42, 41, 41, 41, 42, 41, 41, 41, 41, 41, 41, 42, 42, 42, 41, 45, 46, 41, 40, 41, 41, 42, 41, 41, 41, 41, 41, 41, 40, 41, 43, 40, 40, 40, 40, 43, 41]
Group5:
C.ToList 757.06 ms -> [770, 752, 752, 751, 778, 763, 761, 763, 747, 758, 748, 747, 754, 749, 752, 753, 756, 762, 750, 753, 756, 749, 755, 757, 755, 756, 755, 744, 753, 758, 747, 751, 759, 751, 761, 755, 746, 752, 752, 749, 746, 752, 753, 755, 752, 755, 754, 754, 966, 937, 749, 759, 748, 747, 754, 749, 755, 750, 746, 754, 757, 752, 753, 745, 758, 755, 761, 753, 751, 755, 755, 752, 746, 756, 755, 746, 742, 751, 751, 749, 752, 751, 756, 756, 755, 742, 749, 754, 749, 756, 753, 751, 754, 752, 751, 754, 753, 749, 755, 756]
C.ToArray 772.8 ms -> [766, 772, 755, 763, 758, 767, 763, 762, 761, 768, 769, 763, 770, 757, 765, 760, 766, 759, 764, 761, 760, 777, 1102, 881, 759, 765, 758, 762, 772, 761, 758, 757, 765, 769, 769, 761, 762, 762, 763, 760, 770, 764, 760, 768, 758, 766, 763, 770, 769, 761, 764, 761, 761, 767, 761, 762, 764, 757, 765, 766, 767, 771, 753, 762, 769, 768, 759, 764, 764, 760, 763, 763, 763, 763, 763, 767, 761, 771, 760, 765, 760, 758, 768, 770, 751, 771, 767, 771, 765, 763, 760, 765, 765, 769, 767, 767, 1193, 774, 767, 764]
S.ToList 704.73 ms -> [682, 708, 705, 699, 705, 704, 695, 703, 702, 699, 701, 708, 699, 702, 703, 701, 701, 699, 701, 707, 707, 700, 701, 705, 700, 697, 706, 702, 701, 706, 699, 692, 702, 697, 707, 704, 697, 698, 699, 699, 702, 703, 698, 697, 702, 703, 702, 704, 694, 697, 707, 695, 711, 710, 700, 693, 703, 699, 699, 706, 698, 701, 703, 704, 698, 706, 700, 704, 701, 699, 702, 705, 694, 698, 709, 736, 1053, 704, 694, 700, 698, 696, 701, 700, 700, 706, 706, 692, 698, 707, 703, 695, 703, 699, 694, 708, 695, 694, 706, 695]
S.ToArray 744.17 ms -> [746, 740, 725, 740, 739, 731, 746, 760, 735, 738, 740, 734, 744, 748, 737, 744, 745, 727, 736, 738, 728, 743, 745, 735, 748, 760, 739, 748, 762, 742, 741, 747, 733, 746, 758, 742, 742, 741, 724, 744, 747, 727, 740, 740, 729, 742, 757, 741, 740, 742, 726, 739, 746, 1133, 749, 737, 730, 740, 747, 733, 747, 752, 731, 747, 742, 730, 741, 749, 731, 749, 743, 730, 747, 742, 731, 737, 745, 734, 739, 735, 727, 743, 752, 731, 744, 742, 729, 740, 746, 731, 739, 746, 733, 745, 743, 733, 739, 742, 727, 737]
C1.ToList 31.71 ms -> [35, 32, 32, 30, 31, 33, 31, 32, 32, 31, 31, 32, 32, 33, 32, 31, 31, 32, 31, 32, 32, 32, 31, 32, 33, 32, 31, 31, 31, 32, 31, 34, 31, 31, 32, 33, 32, 32, 31, 32, 34, 32, 31, 32, 33, 31, 32, 32, 31, 32, 32, 32, 32, 32, 32, 31, 31, 32, 31, 33, 30, 31, 32, 30, 30, 33, 32, 32, 34, 31, 31, 31, 31, 32, 31, 31, 31, 31, 32, 31, 31, 33, 31, 32, 32, 32, 33, 32, 31, 31, 31, 31, 31, 32, 32, 33, 32, 31, 31, 32]
C1.ToArray 59.53 ms -> [63, 57, 58, 58, 57, 59, 59, 57, 60, 131, 127, 67, 58, 56, 59, 56, 57, 58, 58, 58, 57, 59, 60, 57, 57, 59, 60, 57, 57, 57, 58, 58, 58, 58, 57, 57, 61, 57, 58, 57, 57, 57, 57, 57, 58, 58, 58, 58, 57, 58, 59, 57, 58, 57, 57, 59, 58, 58, 59, 57, 59, 57, 56, 56, 59, 56, 56, 59, 57, 58, 58, 58, 57, 58, 59, 59, 58, 57, 58, 62, 65, 57, 57, 57, 58, 60, 59, 58, 59, 57, 58, 57, 58, 59, 58, 58, 58, 59, 60, 58]
S1.ToList 82.78 ms -> [87, 82, 83, 83, 82, 82, 83, 84, 82, 83, 84, 84, 84, 82, 82, 84, 82, 84, 83, 84, 82, 82, 82, 81, 83, 83, 83, 84, 84, 82, 82, 83, 83, 83, 82, 83, 85, 83, 82, 82, 84, 82, 82, 83, 83, 83, 82, 82, 82, 83, 82, 83, 82, 84, 82, 83, 82, 83, 82, 82, 82, 84, 82, 83, 82, 82, 86, 83, 83, 82, 83, 83, 83, 82, 84, 82, 83, 81, 82, 82, 82, 82, 83, 83, 83, 82, 83, 84, 83, 82, 83, 83, 83, 82, 83, 84, 82, 82, 83, 83]
S1.ToArray 122.3 ms -> [122, 119, 119, 120, 119, 120, 120, 121, 119, 119, 122, 120, 120, 120, 122, 120, 123, 120, 120, 120, 121, 123, 120, 120, 120, 121, 120, 121, 122, 120, 123, 119, 121, 118, 121, 120, 120, 120, 119, 124, 119, 121, 119, 120, 120, 120, 120, 120, 122, 121, 123, 230, 203, 123, 119, 119, 122, 119, 120, 120, 120, 122, 120, 121, 120, 121, 120, 121, 120, 121, 120, 120, 120, 121, 122, 121, 123, 119, 119, 119, 119, 121, 120, 120, 120, 122, 121, 122, 119, 120, 120, 121, 121, 120, 121, 120, 121, 118, 118, 118]
C2.ToList 3.43 ms -> [5, 3, 4, 4, 4, 3, 4, 4, 4, 4, 4, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 4, 3, 4, 4, 3, 3, 3, 3, 4, 3, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 6, 4, 4, 3, 3, 4, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 4, 3, 4, 4, 4, 3, 4, 4, 3, 4, 4, 4, 4, 4, 4, 3, 3, 3, 4, 4, 3, 3, 3, 3]
C2.ToArray 3.48 ms -> [3, 3, 3, 3, 4, 4, 3, 4, 4, 4, 3, 4, 3, 3, 4, 3, 3, 4, 3, 4, 3, 3, 3, 4, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 3, 4, 3, 3, 3, 3, 4, 3, 3, 3, 3, 4, 3, 4, 3, 3, 4, 3, 3, 4, 3, 3, 4, 4, 3, 3, 4, 3, 4, 4, 3, 4, 4, 4, 4, 4, 3, 3, 3, 4, 4, 3, 4, 4, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 3, 4, 4, 4, 4, 3]
S2.ToList 41.47 ms -> [41, 41, 49, 67, 82, 41, 41, 40, 40, 40, 40, 40, 41, 40, 40, 40, 40, 40, 41, 40, 42, 42, 40, 40, 41, 41, 41, 40, 41, 40, 41, 40, 41, 40, 42, 41, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 41, 41, 41, 41, 41, 42, 41, 41, 41, 42, 40, 41, 40, 40, 40, 42, 40, 41, 42, 41, 42, 41, 42, 40, 41, 41, 41, 41, 41, 41, 41, 41, 40, 41, 40, 41, 41, 41, 40, 41, 41, 40, 40, 41, 41, 41, 41, 41, 43, 40, 40, 41, 42, 41]
S2.ToArray 40.62 ms -> [42, 41, 44, 40, 40, 40, 40, 41, 41, 40, 41, 41, 41, 40, 41, 41, 40, 41, 41, 40, 41, 40, 40, 41, 42, 41, 41, 41, 40, 40, 40, 40, 40, 41, 41, 42, 40, 41, 41, 41, 41, 41, 40, 42, 40, 40, 41, 41, 41, 40, 41, 40, 40, 40, 40, 40, 41, 40, 40, 41, 40, 40, 40, 40, 41, 40, 41, 41, 41, 40, 41, 41, 40, 41, 40, 41, 42, 40, 41, 41, 42, 41, 41, 40, 41, 40, 41, 40, 41, 41, 40, 40, 40, 41, 41, 40, 40, 40, 40, 40]
由于stackoverflow对应答的字符数有限制，因此省略了Group2和Group3的示例列表。
如您所见，在大多数情况下使用ToList或ToArry并不重要。
在处理运行时计算的IEnumerable<T>对象时，如果计算带来的负载比ToList和ToArray的内存分配和复制操作重，则差异不显著(C.ToList vs .ToArray, S.ToList vs .ToArray)。
The difference can be observed only on non-runtime-calculated IEnumerable<T> only objects (C1.ToList vs C1.ToArray and S1.ToList vs S1.ToArray). But the absolute difference (<60ms) is still acceptable on an one million small object IEnumerable<T>. In fact, the difference is decided by the implementation of Enumerator<T> of IEnumerable<T>. So, if your program is really really really sensitive on this, you do have to profile, profile, profile! At last you'll probably find that the bottleneck is not on ToList or ToArray, but the detail of enumerators.
C2的结果。ToList vs C2。ToArray和S2。ToList vs S2。ToArray表明，您真的不需要关心非运行时计算的ICollection<T>对象上的ToList或ToArray。
当然，这只是在我的机器上的结果，这些操作在不同机器上的实际花费时间是不一样的，你可以在你的机器上使用上面的代码来发现。
你需要做出选择的唯一原因是，你在List<T>或T[]上有特定的需求，正如@Jeppe Stig Nielsen的回答所描述的那样。
我同意@mquander的观点，性能差异应该是微不足道的。但是，我想对它进行基准测试，所以我这样做了——结果是微不足道的。
Testing with List<T> source:
ToArray time: 1934 ms (0.01934 ms/call), memory used: 4021 bytes/array
ToList time: 1902 ms (0.01902 ms/call), memory used: 4045 bytes/List
Testing with array source:
ToArray time: 1957 ms (0.01957 ms/call), memory used: 4021 bytes/array
ToList time: 2022 ms (0.02022 ms/call), memory used: 4045 bytes/List
每个源数组/列表有1000个元素。所以你可以看到时间和记忆的差异都可以忽略不计。
我的结论是:您还可以使用ToList()，因为List<T>提供了比数组更多的功能，除非几个字节的内存确实对您很重要。
您应该根据理想的设计选择来决定使用ToList还是ToArray。如果您想要一个只能通过索引迭代和访问的集合，请选择ToArray。如果您希望以后能够轻松地从集合中添加和删除额外的功能，那么可以使用ToList(并不是说您不能添加到数组中，但这通常不是合适的工具)。
如果性能很重要，您还应该考虑哪些操作会更快。实际上，您不会调用ToList或ToArray一百万次，但可能会对获得的集合进行一百万次操作。在这方面[]更好，因为List<>是[]，有一些开销。查看这个线程的一些效率比较:List<int>或int[]
在我自己不久前的测试中，我发现ToArray更快。我不确定这些测试有多偏颇。然而，性能差异是如此微不足道，只有在循环运行这些查询数百万次时才能明显看出。
