Reimplement LINQ ToList using SegmentedArrayBuilder to reduce allocations

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!

[andrewjsaid]

After #96570, ToArray() in many places is faster and has fewer allocations that ToList(). As a List is just a wrapper over an array, we can re-use most of the logic in SegmentedArrayBuilder to build Lists, too.

There were some more complex cases using SegmentedArrayBuilder for example Concat where I left the code as-is. I can also experiment with performance numbers for those if it is requested.

Benchmarks

Enumerable_ToList changes have been reverted, they are not in current PR version

Int32Tests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	25.87 ns	1.00	112 B	1.00
Enumerable_ToList	pr	1	28.49 ns	1.10	104 B	0.93
IEnumerableSelectIterator_ToList	main	1	46.05 ns	1.00	168 B	1.00
IEnumerableSelectIterator_ToList	pr	1	48.76 ns	1.06	160 B	0.95
IteratorSelectIterator_ToList	main	1	55.84 ns	1.00	184 B	1.00
IteratorSelectIterator_ToList	pr	1	60.04 ns	1.08	184 B	1.00
OfTypeIterator_ToList	main	1	61.88 ns	1.00	200 B	1.00
OfTypeIterator_ToList	pr	1	65.08 ns	1.05	176 B	0.88
SelectManySingleSelectorIterator_ToList	main	1	62.16 ns	1.00	240 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	67.54 ns	1.08	232 B	0.97
IEnumerableSkipTakeIterator_ToLost	main	1	41.68 ns	1.00	128 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	43.83 ns	1.05	128 B	1.00
IEnumerableWhereIterator_ToList	main	1	42.11 ns	1.00	168 B	1.00
IEnumerableWhereIterator_ToList	pr	1	42.63 ns	1.01	160 B	0.95
ArrayWhereIterator_ToList	main	1	38.62 ns	1.00	152 B	1.00
ArrayWhereIterator_ToList	pr	1	45.68 ns	1.18	144 B	0.95
IEnumerableWhereSelectIterator_ToList	main	1	60.61 ns	1.00	232 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	65.19 ns	1.10	224 B	0.97
ArrayWhereSelectIterator_ToList	main	1	55.79 ns	1.00	208 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	60.01 ns	1.07	200 B	0.96
Enumerable_ToList	main	5	44.33 ns	1.00	168 B	1.00
Enumerable_ToList	pr	5	32.99 ns	0.74	120 B	0.71
IEnumerableSelectIterator_ToList	main	5	65.96 ns	1.00	224 B	1.00
IEnumerableSelectIterator_ToList	pr	5	56.28 ns	0.85	176 B	0.79
IteratorSelectIterator_ToList	main	5	73.91 ns	1.00	224 B	1.00
IteratorSelectIterator_ToList	pr	5	77.94 ns	1.05	224 B	1.00
OfTypeIterator_ToList	main	5	132.67 ns	1.00	384 B	1.00
OfTypeIterator_ToList	pr	5	115.63 ns	0.87	304 B	0.79
SelectManySingleSelectorIterator_ToList	main	5	122.24 ns	1.00	296 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	106.91 ns	0.88	248 B	0.84
IEnumerableSkipTakeIterator_ToLost	main	5	56.19 ns	1.00	168 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	58.61 ns	1.04	168 B	1.00
IEnumerableWhereIterator_ToList	main	5	62.12 ns	1.00	224 B	1.00
IEnumerableWhereIterator_ToList	pr	5	50.66 ns	0.81	176 B	0.79
ArrayWhereIterator_ToList	main	5	56.66 ns	1.00	224 B	1.00
ArrayWhereIterator_ToList	pr	5	49.61 ns	0.88	176 B	0.79
IEnumerableWhereSelectIterator_ToList	main	5	82.31 ns	1.00	288 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	72.20 ns	0.88	240 B	0.83
ArrayWhereSelectIterator_ToList	main	5	74.32 ns	1.00	280 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	62.94 ns	0.85	232 B	0.83
Enumerable_ToList	main	50	167.20 ns	1.00	688 B	1.00
Enumerable_ToList	pr	50	118.14 ns	0.71	296 B	0.43
IEnumerableSelectIterator_ToList	main	50	191.60 ns	1.00	744 B	1.00
IEnumerableSelectIterator_ToList	pr	50	171.93 ns	0.90	352 B	0.47
IteratorSelectIterator_ToList	main	50	251.54 ns	1.00	800 B	1.00
IteratorSelectIterator_ToList	pr	50	231.18 ns	0.92	408 B	0.51
OfTypeIterator_ToList	main	50	731.40 ns	1.00	2432 B	1.00
OfTypeIterator_ToList	pr	50	734.97 ns	1.01	1744 B	0.72
SelectManySingleSelectorIterator_ToList	main	50	692.19 ns	1.00	816 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	656.03 ns	0.95	424 B	0.52
IEnumerableSkipTakeIterator_ToLost	main	50	186.72 ns	1.00	744 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	161.33 ns	0.86	352 B	0.47
IEnumerableWhereIterator_ToList	main	50	187.08 ns	1.00	744 B	1.00
IEnumerableWhereIterator_ToList	pr	50	164.58 ns	0.88	352 B	0.47
ArrayWhereIterator_ToList	main	50	154.86 ns	1.00	920 B	1.00
ArrayWhereIterator_ToList	pr	50	141.83 ns	0.92	528 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	209.25 ns	1.00	808 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	186.49 ns	0.89	416 B	0.51
ArrayWhereSelectIterator_ToList	main	50	171.83 ns	1.00	976 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	145.71 ns	0.85	584 B	0.60

ObjectTests

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.302
[Host] : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
Job-YPZLVD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
Job-OASBAG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2
LaunchCount=3 

Method		Count	Mean	Ratio	Allocated	Alloc Ratio
Enumerable_ToList	main	1	41.15 ns	1.00	136 B	1.00
Enumerable_ToList	pr	1	45.14 ns	1.10	112 B	0.82
IEnumerableSelectIterator_ToList	main	1	68.84 ns	1.00	192 B	1.00
IEnumerableSelectIterator_ToList	pr	1	68.88 ns	1.01	168 B	0.88
IteratorSelectIterator_ToList	main	1	75.90 ns	1.00	192 B	1.00
IteratorSelectIterator_ToList	pr	1	84.56 ns	1.11	192 B	1.00
OfTypeIterator_ToList	main	1	57.58 ns	1.00	184 B	1.00
OfTypeIterator_ToList	pr	1	61.52 ns	1.07	160 B	0.87
SelectManySingleSelectorIterator_ToList	main	1	88.42 ns	1.00	264 B	1.00
SelectManySingleSelectorIterator_ToList	pr	1	89.23 ns	1.00	240 B	0.91
IEnumerableSkipTakeIterator_ToLost	main	1	55.73 ns	1.00	136 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	1	60.91 ns	1.09	136 B	1.00
IEnumerableWhereIterator_ToList	main	1	66.58 ns	1.00	192 B	1.00
IEnumerableWhereIterator_ToList	pr	1	70.11 ns	1.05	168 B	0.88
ArrayWhereIterator_ToList	main	1	48.82 ns	1.00	168 B	1.00
ArrayWhereIterator_ToList	pr	1	56.08 ns	1.15	144 B	0.86
IEnumerableWhereSelectIterator_ToList	main	1	87.87 ns	1.00	256 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	1	90.83 ns	1.03	232 B	0.91
ArrayWhereSelectIterator_ToList	main	1	70.01 ns	1.00	224 B	1.00
ArrayWhereSelectIterator_ToList	pr	1	73.59 ns	1.05	200 B	0.89
Enumerable_ToList	main	5	84.91 ns	1.00	224 B	1.00
Enumerable_ToList	pr	5	71.13 ns	0.83	144 B	0.64
IEnumerableSelectIterator_ToList	main	5	168.50 ns	1.00	280 B	1.00
IEnumerableSelectIterator_ToList	pr	5	153.26 ns	0.91	200 B	0.71
IteratorSelectIterator_ToList	main	5	121.77 ns	1.00	248 B	1.00
IteratorSelectIterator_ToList	pr	5	187.19 ns	1.54	248 B	1.00
OfTypeIterator_ToList	main	5	101.74 ns	1.00	272 B	1.00
OfTypeIterator_ToList	pr	5	88.26 ns	0.87	192 B	0.71
SelectManySingleSelectorIterator_ToList	main	5	187.48 ns	1.00	352 B	1.00
SelectManySingleSelectorIterator_ToList	pr	5	153.98 ns	0.82	272 B	0.77
IEnumerableSkipTakeIterator_ToLost	main	5	91.36 ns	1.00	192 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	5	94.57 ns	1.04	192 B	1.00
IEnumerableWhereIterator_ToList	main	5	170.63 ns	1.00	280 B	1.00
IEnumerableWhereIterator_ToList	pr	5	154.21 ns	0.90	200 B	0.71
ArrayWhereIterator_ToList	main	5	144.74 ns	1.00	288 B	1.00
ArrayWhereIterator_ToList	pr	5	135.21 ns	0.94	208 B	0.72
IEnumerableWhereSelectIterator_ToList	main	5	191.12 ns	1.00	344 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	5	176.70 ns	0.93	264 B	0.77
ArrayWhereSelectIterator_ToList	main	5	166.66 ns	1.00	344 B	1.00
ArrayWhereSelectIterator_ToList	pr	5	149.40 ns	0.90	264 B	0.77
Enumerable_ToList	main	50	452.81 ns	1.00	1192 B	1.00
Enumerable_ToList	pr	50	429.92 ns	0.94	504 B	0.42
IEnumerableSelectIterator_ToList	main	50	543.63 ns	1.00	1248 B	1.00
IEnumerableSelectIterator_ToList	pr	50	552.12 ns	1.02	560 B	0.45
IteratorSelectIterator_ToList	main	50	701.63 ns	1.00	1304 B	1.00
IteratorSelectIterator_ToList	pr	50	707.15 ns	1.01	608 B	0.47
OfTypeIterator_ToList	main	50	499.60 ns	1.00	1240 B	1.00
OfTypeIterator_ToList	pr	50	501.70 ns	1.00	552 B	0.45
SelectManySingleSelectorIterator_ToList	main	50	1,267.24 ns	1.00	1320 B	1.00
SelectManySingleSelectorIterator_ToList	pr	50	1,033.94 ns	0.82	632 B	0.48
IEnumerableSkipTakeIterator_ToLost	main	50	475.09 ns	1.00	1248 B	1.00
IEnumerableSkipTakeIterator_ToLost	pr	50	483.19 ns	1.02	552 B	0.44
IEnumerableWhereIterator_ToList	main	50	566.51 ns	1.00	1248 B	1.00
IEnumerableWhereIterator_ToList	pr	50	559.30 ns	0.99	560 B	0.45
ArrayWhereIterator_ToList	main	50	474.29 ns	1.00	1616 B	1.00
ArrayWhereIterator_ToList	pr	50	544.54 ns	1.15	928 B	0.57
IEnumerableWhereSelectIterator_ToList	main	50	581.70 ns	1.00	1312 B	1.00
IEnumerableWhereSelectIterator_ToList	pr	50	599.43 ns	1.03	624 B	0.48
ArrayWhereSelectIterator_ToList	main	50	494.47 ns	1.00	1672 B	1.00
ArrayWhereSelectIterator_ToList	pr	50	499.22 ns	1.01	984 B	0.59

Benchmark Code

Command Line

dotnet run -c Release `--corerun `
D:\Code\_forks\dotnet-runtime-base\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`
D:\Code\_forks\dotnet-runtime\artifacts\bin\testhost\net9.0-windows-Release-x64\shared\Microsoft.NETCore.App\9.0.0\corerun.exe`--launchCount 3

Program.cs

usingBenchmarkDotNet.Attributes;usingBenchmarkDotNet.Running;BenchmarkSwitcher.FromAssembly(typeof(Tests<>).Assembly).Run(args);publicpartialclassTests<T>whereT:new(){privateT_value=new();privateT[]_valueArr=[new()];privateIEnumerable<T>CreateEnumerable(){for(inti=0;i<Count;i++)yieldreturn_value;}privateIEnumerable<T>CreateArray(){varresult=newT[Count];for(inti=0;i<Count;i++){result[i]=_value;}returnresult;}[Params(1,5,50)]publicintCount{get;set;}[Benchmark]publicList<T>Enumerable_ToList()=>CreateEnumerable().ToList();[Benchmark]publicList<T>IEnumerableSelectIterator_ToList()=>AssertType(CreateEnumerable().Select(i =>i),"IEnumerableSelectIterator`2").ToList();[Benchmark]publicList<T>IteratorSelectIterator_ToList()=>AssertType(CreateEnumerable().Skip(1).Select(i =>i),"IteratorSelectIterator`2").ToList();[Benchmark]publicList<object>OfTypeIterator_ToList()=>AssertType(CreateEnumerable().OfType<object>(),"OfTypeIterator`1").ToList();[Benchmark]publicList<T>SelectManySingleSelectorIterator_ToList()=>AssertType(CreateEnumerable().SelectMany(i =>_valueArr),"SelectManySingleSelectorIterator`2").ToList();[Benchmark]publicList<T>IEnumerableSkipTakeIterator_ToLost()=>AssertType(CreateEnumerable().Skip(1),"IEnumerableSkipTakeIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true),"IEnumerableWhereIterator`1").ToList();[Benchmark]publicList<T>ArrayWhereIterator_ToList()=>AssertType(CreateArray().Where(_ =>true),"ArrayWhereIterator`1").ToList();[Benchmark]publicList<T>IEnumerableWhereSelectIterator_ToList()=>AssertType(CreateEnumerable().Where(_ =>true).Select(i =>i),"IEnumerableWhereSelectIterator`2").ToList();[Benchmark]publicList<T>ArrayWhereSelectIterator_ToList()=>AssertType(CreateArray().Where(_ =>true).Select(i =>i),"ArrayWhereSelectIterator`2").ToList();privatestaticTIteratorAssertType<TIterator>(TIteratorvalue,stringtype){if(value!.GetType().Nameis{}actual&&actual!=type){thrownewInvalidOperationException($"Expected {type} got {actual}");}returnvalue;}}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassInt32Tests:Tests<int>{}[HideColumns("Error","StdDev","Median","RatioSD","Job")][MemoryDiagnoser(false)]publicclassObjectTests:Tests<object>{}

Performance Analysis

In all cases allocations are down and sometimes significantly so. As the size of the collection increases so do the savings due to reduced number of "unnecessary" array copies as the list grows.

Speed is more of a trade-off. For lower counts the PR is actually slower whereas for higher counts it is faster, again due to less time copying data around.

However this must be seen in context; we are trading off nanoseconds. In situations where nanoseconds of individual LINQ operations matter, it would probably be recommended to write the loops by hand anyway. On this basis I would say that creating less garbage to collect would probably be better for the overall system performance than the slower code for the Count less than 5 case.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

[stephentoub]

Thanks.

[eiriktsarpalis]

Thanks!