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
除非您只是需要一个数组来满足其他约束，否则您应该使用ToList。在大多数情况下，ToArray会比ToList分配更多的内存。
两者都使用数组进行存储，但是ToList有一个更灵活的约束。它需要数组至少与集合中的元素数量一样大。如果数组更大，这不是问题。但是ToArray需要数组的大小精确到元素的数量。
为了满足这个约束，ToArray通常比ToList多做一次分配。一旦它有了一个足够大的数组，它就会分配一个完全正确大小的数组，并将元素复制回该数组中。唯一可以避免这种情况的情况是当数组的增长算法恰好与需要存储的元素数量一致时(绝对是少数)。
EDIT
有几个人问我在List<T>值中有额外的未使用内存的后果。
这是一个合理的担忧。如果创建的集合寿命很长，在创建后从未被修改过，并且有很高的机会落在Gen2堆中，那么您可能会更好地预先分配额外的ToArray。
总的来说，我发现这种情况比较罕见。更常见的情况是，大量ToArray调用被立即传递给其他短期内存使用，在这种情况下，ToList显然更好。
这里的关键是分析，分析，再分析更多。
其他回答
一种选择是添加自己的扩展方法，该方法返回一个只读的ICollection<T>。当您既不想使用数组/列表的索引属性，也不想从列表中添加/删除时，这可能比使用ToList或ToArray更好。
public static class EnumerableExtension
{
/// <summary>
/// Causes immediate evaluation of the linq but only if required.
/// As it returns a readonly ICollection, is better than using ToList or ToArray
/// when you do not want to use the indexing properties of an IList, or add to the collection.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="enumerable"></param>
/// <returns>Readonly collection</returns>
public static ICollection<T> Evaluate<T>(this IEnumerable<T> enumerable)
{
//if it's already a readonly collection, use it
var collection = enumerable as ICollection<T>;
if ((collection != null) && collection.IsReadOnly)
{
return collection;
}
//or make a new collection
return enumerable.ToList().AsReadOnly();
}
}
单元测试:
[TestClass]
public sealed class EvaluateLinqTests
{
[TestMethod]
public void EvalTest()
{
var list = new List<int> {1, 2, 3};
var linqResult = list.Select(i => i);
var linqResultEvaluated = list.Select(i => i).Evaluate();
list.Clear();
Assert.AreEqual(0, linqResult.Count());
//even though we have cleared the underlying list, the evaluated list does not change
Assert.AreEqual(3, linqResultEvaluated.Count());
}
[TestMethod]
public void DoesNotSaveCreatingListWhenHasListTest()
{
var list = new List<int> {1, 2, 3};
var linqResultEvaluated = list.Evaluate();
//list is not readonly, so we expect a new list
Assert.AreNotSame(list, linqResultEvaluated);
}
[TestMethod]
public void SavesCreatingListWhenHasReadonlyListTest()
{
var list = new List<int> {1, 2, 3}.AsReadOnly();
var linqResultEvaluated = list.Evaluate();
//list is readonly, so we don't expect a new list
Assert.AreSame(list, linqResultEvaluated);
}
[TestMethod]
public void SavesCreatingListWhenHasArrayTest()
{
var list = new[] {1, 2, 3};
var linqResultEvaluated = list.Evaluate();
//arrays are readonly (wrt ICollection<T> interface), so we don't expect a new object
Assert.AreSame(list, linqResultEvaluated);
}
[TestMethod]
[ExpectedException(typeof (NotSupportedException))]
public void CantAddToResultTest()
{
var list = new List<int> {1, 2, 3};
var linqResultEvaluated = list.Evaluate();
Assert.AreNotSame(list, linqResultEvaluated);
linqResultEvaluated.Add(4);
}
[TestMethod]
[ExpectedException(typeof (NotSupportedException))]
public void CantRemoveFromResultTest()
{
var list = new List<int> {1, 2, 3};
var linqResultEvaluated = list.Evaluate();
Assert.AreNotSame(list, linqResultEvaluated);
linqResultEvaluated.Remove(1);
}
}
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。
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。
除非您只是需要一个数组来满足其他约束，否则您应该使用ToList。在大多数情况下，ToArray会比ToList分配更多的内存。
两者都使用数组进行存储，但是ToList有一个更灵活的约束。它需要数组至少与集合中的元素数量一样大。如果数组更大，这不是问题。但是ToArray需要数组的大小精确到元素的数量。
为了满足这个约束，ToArray通常比ToList多做一次分配。一旦它有了一个足够大的数组，它就会分配一个完全正确大小的数组，并将元素复制回该数组中。唯一可以避免这种情况的情况是当数组的增长算法恰好与需要存储的元素数量一致时(绝对是少数)。
EDIT
有几个人问我在List<T>值中有额外的未使用内存的后果。
这是一个合理的担忧。如果创建的集合寿命很长，在创建后从未被修改过，并且有很高的机会落在Gen2堆中，那么您可能会更好地预先分配额外的ToArray。
总的来说，我发现这种情况比较罕见。更常见的情况是，大量ToArray调用被立即传递给其他短期内存使用，在这种情况下，ToList显然更好。
这里的关键是分析，分析，再分析更多。
一个很晚的答案，但我认为这对谷歌人有帮助。
They both suck when they created using linq. They both implement same code to resize buffer if necessary. ToArray internally uses a class to convert IEnumerable<> to array, by allocating an array of 4 elements. If that is not enough than it doubles the size by creating a new array double the size of current and copying current array to it. At the end it allocates a new array of count of your items. If your query returns 129 elements then ToArray will make 6 allocations and memory copy operations to create a 256 element array and than am another array of 129 to return. so much for memory efficiency.
ToList做同样的事情，但是它跳过了最后的分配，因为您可以在将来添加项。List不关心它是从linq查询创建的还是手动创建的。
List在内存上更好，但在cpu上更差，因为List是一个通用的解决方案，每个操作都需要范围检查，除了.net内部的数组范围检查之外。
因此，如果你将迭代你的结果集太多次，那么数组是很好的，因为它意味着比列表更少的范围检查，编译器通常优化数组的顺序访问。
如果在创建List时指定capacity参数，则它的初始化分配可以更好。在这种情况下，它将只分配数组一次，假设您知道结果大小。linq的ToList没有指定重载来提供它，因此我们必须创建扩展方法，该方法创建一个具有给定容量的列表，然后使用list <>. addrange。
为了完成这个问题，我必须写出下面的句子
At the end, you can use either an ToArray, or ToList, performance will not be so different ( see answer of @EMP ). You are using C#. If you need performance then do not worry about writing about high performance code, but worry about not writing bad performance code. Always target x64 for high performance code. AFAIK, x64 JIT is based on C++ compiler, and does some funny things like tail recursion optimizations. With 4.5 you can also enjoy the profile guided optimization and multi core JIT. At last, you can use async/await pattern to process it quicker.
