C# Collection Benchmarks

One of the most fundamental questions developers must ask themselves repeatedly when coding is: which data structure should I use? However, more subtle questions that are just as important include:

• Which type should I use for that data structure?
• Do I need to create defensive copies for safety?
• What's the difference between read-only collections and immutable collections, and which should I use?
• Do my answers change if I'm dealing with concurrent access or modification?

An extremely common scenario in many programs is to create a collection of items as an output of some operation and then pass that collection off to some other routine for consumption. In this situation, creating/mutating the collection is done once, and all consumption of the collection after that point is read-only. For example, maybe you retrieve items from a DB, filter some of them down to a smaller set, transform them into JSON, and then return that JSON to an application. In C#, this could be done in a couple ways:

var dbItems = db.GetItems();
var filteredItems = filter(dbItems);
var asJson = convertToJson(filteredItems);
return asJson;

Or, more simply, using LINQ: return convertToJson(db.GetItems().Where(filter));

What type should db.GetItems() return? What types should filter() accept and return? How about convertToJson()? The answers to those questions can have varying impacts to safety and performance that are often not clear. Additionally, those coming from a Java background may be surprised to learn how things work in C#, where the best answer in Java is not the best answer in C#.

This project contains microbenchmarks that are meant to help answer these questions.

For Those Coming from Java

As an aside, before getting to the C# benchmarks below, a quick note about the difference between the .NET immutable collections and the Guava immutable collections often used in Java development, since these are quite different. The Guava immutable collections disable mutation and are meant for very fast and efficient consumption, whether single-threaded or multi-threaded. The .NET immutable collections, however, enable mutation but do so in such a way that the modification is efficiently returned as a separate collection instance. This difference makes the Guava immutable collections appropriate for consumption but not production, while the .NET immutable collections are appropriate for thread-safe production but not necessarily efficient consumption.

Benchmarks

All benchmarks in this project use BenchmarkDotNet and were executed on a MacBook Pro 2019 16-inch model, plugged in, with no activities other than the benchmarks.

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Iterate list

Arguably the most common pattern you'll find in a program is to create a list of items in a single thread and then iterate over those unchanging items. The included IterationBenchy tests the performance and memory characteristics of various collection types when used in this pattern for both a list of 100 items and a list of 1000 items.

Benchmark Results

BenchmarkDotNet v0.13.12, macOS Sonoma 14.4.1 (23E224) [Darwin 23.4.0]

Intel Core i9-9980HK CPU 2.40GHz, 1 CPU, 16 logical and 8 physical cores

.NET SDK=8.0.101
Job-VFPNOY : .NET Core 3.1.20 (CoreCLR 4.700.21.47003, CoreFX 4.700.21.47101), X64 RyuJIT AVX2
Job-VZMYMY : .NET 5.0.11 (5.0.1121.47308), X64 RyuJIT AVX2
Job-OVRMSJ : .NET 6.0.8 (6.0.822.36306), X64 RyuJIT AVX2
Job-FDUCSD : .NET 7.0.0 (7.0.22.51805), X64 RyuJIT AVX2
Job-HTCTKA : .NET 8.0.1 (8.0.123.58001), X64 RyuJIT AVX2

Method	Runtime	N	Mean	Error	StdDev	Allocated
IterateAsSpan	.NET Core 3.1	N/A	N/A	N/A	N/A	-
IterateAsSpan	.NET 5.0	N/A	N/A	N/A	N/A	-
IterateAsSpan	.NET 6.0	100	33.11 ns	0.674 ns	0.776 ns	-
IterateAsSpan	.NET 7.0	100	32.30 ns	0.652 ns	0.610 ns	-
IterateAsSpan	.NET 8.0	100	31.98 ns	0.665 ns	0.932 ns	-
IterateAsMutableList	.NET Core 3.1	100	333.9 ns	2.61 ns	2.44 ns	-
IterateAsMutableList	.NET 5.0	100	305.71 ns	6.064 ns	6.227 ns	-
IterateAsMutableList	.NET 6.0	100	97.04 ns	1.873 ns	2.004 ns	-
IterateAsMutableList	.NET 7.0	100	79.60 ns	1.551 ns	1.787 ns	-
IterateAsMutableList	.NET 8.0	100	54.62 ns	0.796 ns	0.706 ns	-
IterateAsMutableIList	.NET Core 3.1	100	880.0 ns	4.86 ns	3.79 ns	40 B
IterateAsMutableIList	.NET 5.0	100	835.10 ns	13.057 ns	12.213 ns	40 B
IterateAsMutableIList	.NET 6.0	100	975.59 ns	19.508 ns	20.874 ns	40 B
IterateAsMutableIList	.NET 7.0	100	901.60 ns	17.241 ns	16.127 ns	40 B
IterateAsMutableIList	.NET 8.0	100	281.74 ns	5.329 ns	5.472 ns	40 B
IterateAsMutableICollection	.NET Core 3.1	100	897.0 ns	7.09 ns	6.63 ns	40 B
IterateAsMutableICollection	.NET 5.0	100	829.72 ns	15.439 ns	14.441 ns	40 B
IterateAsMutableICollection	.NET 6.0	100	1,021.42 ns	20.238 ns	48.097 ns	40 B
IterateAsMutableICollection	.NET 7.0	100	894.68 ns	13.823 ns	12.930 ns	40 B
IterateAsMutableICollection	.NET 8.0	100	281.64 ns	5.578 ns	6.640 ns	40 B
IterateAsIEnumerable	.NET Core 3.1	100	888.0 ns	8.07 ns	7.15 ns	40 B
IterateAsIEnumerable	.NET 5.0	100	835.58 ns	14.329 ns	13.403 ns	40 B
IterateAsIEnumerable	.NET 6.0	100	1,007.00 ns	19.832 ns	27.801 ns	40 B
IterateAsIEnumerable	.NET 7.0	100	903.82 ns	17.124 ns	19.033 ns	40 B
IterateAsIEnumerable	.NET 8.0	100	278.81 ns	5.309 ns	6.320 ns	40 B
IterateAsMutableArray	.NET Core 3.1	100	156.3 ns	1.71 ns	1.43 ns	824 B
IterateAsMutableArray	.NET 5.0	100	123.27 ns	2.482 ns	5.602 ns	824 B
IterateAsMutableArray	.NET 6.0	100	145.39 ns	4.062 ns	11.912 ns	824 B
IterateAsMutableArray	.NET 7.0	100	129.07 ns	2.602 ns	5.136 ns	824 B
IterateAsMutableArray	.NET 8.0	100	105.36 ns	2.106 ns	3.087 ns	824 B
IterateAsIReadOnlyCollection	.NET Core 3.1	100	835.7 ns	7.41 ns	6.94 ns	64 B
IterateAsIReadOnlyCollection	.NET 5.0	100	801.34 ns	15.660 ns	17.406 ns	64 B
IterateAsIReadOnlyCollection	.NET 6.0	100	928.58 ns	16.061 ns	13.412 ns	64 B
IterateAsIReadOnlyCollection	.NET 7.0	100	879.08 ns	17.478 ns	29.679 ns	64 B
IterateAsIReadOnlyCollection	.NET 8.0	100	345.50 ns	6.365 ns	6.536 ns	64 B
IterateAsReadOnlyArray	.NET Core 3.1	100	548.7 ns	6.07 ns	5.38 ns	880 B
IterateAsReadOnlyArray	.NET 5.0	100	548.88 ns	10.817 ns	19.506 ns	880 B
IterateAsReadOnlyArray	.NET 6.0	100	539.32 ns	10.649 ns	14.576 ns	880 B
IterateAsReadOnlyArray	.NET 7.0	100	493.62 ns	9.862 ns	17.530 ns	880 B
IterateAsReadOnlyArray	.NET 8.0	100	233.54 ns	4.650 ns	9.393 ns	880 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET Core 3.1	100	10,080.3 ns	191.44 ns	179.07 ns	4920 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 5.0	100	7,970.88 ns	157.837 ns	147.641 ns	4920 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 6.0	100	6,844.65 ns	124.806 ns	104.219 ns	4920 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 7.0	100	6,699.77 ns	110.707 ns	123.051 ns	4920 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 8.0	100	3,339.33 ns	65.041 ns	57.657 ns	4920 B
IterateAsIImmutableList	.NET Core 3.1	100	9,827.8 ns	61.13 ns	51.05 ns	4920 B
IterateAsIImmutableList	.NET 5.0	100	7,752.44 ns	108.453 ns	96.141 ns	4920 B
IterateAsIImmutableList	.NET 6.0	100	6,845.22 ns	76.360 ns	67.692 ns	4920 B
IterateAsIImmutableList	.NET 7.0	100	6,660.65 ns	132.590 ns	157.839 ns	4920 B
IterateAsIImmutableList	.NET 8.0	100	3,438.22 ns	61.692 ns	57.707 ns	4920 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET Core 3.1	100	614.9 ns	7.40 ns	6.92 ns	880 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 5.0	100	632.24 ns	12.520 ns	17.137 ns	880 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 6.0	100	642.56 ns	12.880 ns	17.194 ns	880 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 7.0	100	598.70 ns	11.597 ns	18.056 ns	880 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 8.0	100	278.27 ns	5.560 ns	10.845 ns	880 B
IterateAsImmutableArray	.NET Core 3.1	100	149.6 ns	2.69 ns	2.52 ns	824 B
IterateAsImmutableArray	.NET 5.0	100	129.41 ns	2.620 ns	5.751 ns	824 B
IterateAsImmutableArray	.NET 6.0	100	130.14 ns	2.593 ns	4.404 ns	824 B
IterateAsImmutableArray	.NET 7.0	100	122.71 ns	2.472 ns	4.994 ns	824 B
IterateAsImmutableArray	.NET 8.0	100	114.83 ns	2.294 ns	3.062 ns	824 B
IterateAsSpan	.NET Core 3.1	N/A	N/A	N/A	N/A	-
IterateAsSpan	.NET 5.0	N/A	N/A	N/A	N/A	-
IterateAsSpan	.NET 6.0	1000	245.31 ns	4.869 ns	9.496 ns	-
IterateAsSpan	.NET 7.0	1000	246.00 ns	4.897 ns	6.014 ns	-
IterateAsSpan	.NET 8.0	1000	246.96 ns	4.966 ns	6.457 ns	-
IterateAsMutableList	.NET Core 3.1	1000	3,168.5 ns	46.34 ns	43.35 ns	-
IterateAsMutableList	.NET 5.0	1000	3,064.51 ns	52.985 ns	49.562 ns	-
IterateAsMutableList	.NET 6.0	1000	872.42 ns	15.731 ns	16.832 ns	-
IterateAsMutableList	.NET 7.0	1000	716.98 ns	14.367 ns	15.969 ns	-
IterateAsMutableList	.NET 8.0	1000	484.34 ns	9.332 ns	17.980 ns	-
IterateAsMutableIList	.NET Core 3.1	1000	8,575.3 ns	49.48 ns	38.63 ns	40 B
IterateAsMutableIList	.NET 5.0	1000	8,149.08 ns	160.072 ns	249.213 ns	40 B
IterateAsMutableIList	.NET 6.0	1000	8,896.33 ns	169.103 ns	173.656 ns	40 B
IterateAsMutableIList	.NET 7.0	1000	8,384.64 ns	153.449 ns	220.072 ns	40 B
IterateAsMutableIList	.NET 8.0	1000	3,106.39 ns	60.543 ns	59.461 ns	40 B
IterateAsMutableICollection	.NET Core 3.1	1000	8,600.0 ns	80.31 ns	71.19 ns	40 B
IterateAsMutableICollection	.NET 5.0	1000	8,092.11 ns	151.778 ns	155.865 ns	40 B
IterateAsMutableICollection	.NET 6.0	1000	8,908.13 ns	173.299 ns	248.540 ns	40 B
IterateAsMutableICollection	.NET 7.0	1000	8,437.38 ns	166.475 ns	340.063 ns	40 B
IterateAsMutableICollection	.NET 8.0	1000	3,105.78 ns	60.999 ns	67.800 ns	40 B
IterateAsIEnumerable	.NET Core 3.1	1000	8,610.6 ns	77.57 ns	68.76 ns	40 B
IterateAsIEnumerable	.NET 5.0	1000	8,094.28 ns	147.501 ns	196.909 ns	40 B
IterateAsIEnumerable	.NET 6.0	1000	8,953.89 ns	174.394 ns	261.025 ns	40 B
IterateAsIEnumerable	.NET 7.0	1000	8,324.44 ns	150.546 ns	251.529 ns	40 B
IterateAsIEnumerable	.NET 8.0	1000	3,076.62 ns	59.929 ns	64.123 ns	40 B
IterateAsMutableArray	.NET Core 3.1	1000	808.1 ns	9.81 ns	8.19 ns	8024 B
IterateAsMutableArray	.NET 5.0	1000	746.47 ns	14.822 ns	18.745 ns	8024 B
IterateAsMutableArray	.NET 6.0	1000	791.07 ns	13.542 ns	18.537 ns	8024 B
IterateAsMutableArray	.NET 7.0	1000	781.98 ns	15.651 ns	21.940 ns	8024 B
IterateAsMutableArray	.NET 8.0	1000	743.54 ns	14.167 ns	12.559 ns	8024 B
IterateAsIReadOnlyCollection	.NET Core 3.1	1000	8,200.6 ns	127.99 ns	119.72 ns	64 B
IterateAsIReadOnlyCollection	.NET 5.0	1000	7,573.45 ns	113.343 ns	94.646 ns	64 B
IterateAsIReadOnlyCollection	.NET 6.0	1000	9,005.13 ns	179.990 ns	252.321 ns	64 B
IterateAsIReadOnlyCollection	.NET 7.0	1000	8,668.80 ns	162.846 ns	305.864 ns	64 B
IterateAsIReadOnlyCollection	.NET 8.0	1000	3,070.69 ns	60.126 ns	76.040 ns	64 B
IterateAsReadOnlyArray	.NET Core 3.1	1000	5,076.5 ns	64.05 ns	56.78 ns	8080 B
IterateAsReadOnlyArray	.NET 5.0	1000	4,867.89 ns	96.912 ns	156.496 ns	8080 B
IterateAsReadOnlyArray	.NET 6.0	1000	4,929.40 ns	97.659 ns	160.457 ns	8080 B
IterateAsReadOnlyArray	.NET 7.0	1000	4,475.06 ns	86.615 ns	134.849 ns	8080 B
IterateAsReadOnlyArray	.NET 8.0	1000	1,948.07 ns	38.853 ns	93.835 ns	8080 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET Core 3.1	1000	99,576.8 ns	1,438.09 ns	1,274.83 ns	48120 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 5.0	1000	74,719.02 ns	1,458.366 ns	1,844.366 ns	48120 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 6.0	1000	62,614.81 ns	1,240.556 ns	1,327.381 ns	48120 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 7.0	1000	65,955.88 ns	1,314.931 ns	1,565.332 ns	48120 B
IterateAsImmutableListCastToIReadOnlyCollection	.NET 8.0	1000	33,799.11 ns	673.462 ns	1,464.052 ns	48120 B
IterateAsIImmutableList	.NET Core 3.1	1000	98,902.6 ns	1,153.25 ns	1,022.33 ns	48121 B
IterateAsIImmutableList	.NET 5.0	1000	74,918.27 ns	1,493.586 ns	2,411.862 ns	48120 B
IterateAsIImmutableList	.NET 6.0	1000	65,774.12 ns	1,305.544 ns	2,753.835 ns	48120 B
IterateAsIImmutableList	.NET 7.0	1000	67,807.60 ns	1,284.156 ns	2,411.956 ns	48120 B
IterateAsIImmutableList	.NET 8.0	1000	31,823.79 ns	632.937 ns	1,173.190 ns	48120 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET Core 3.1	1000	5,311.7 ns	68.45 ns	60.68 ns	8080 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 5.0	1000	5,600.76 ns	108.667 ns	141.298 ns	8080 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 6.0	1000	5,855.68 ns	108.634 ns	159.235 ns	8080 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 7.0	1000	5,435.78 ns	106.556 ns	130.861 ns	8080 B
IterateAsImmutableArrayCastToIReadOnlyCollection	.NET 8.0	1000	2,516.06 ns	49.547 ns	62.661 ns	8080 B
IterateAsImmutableArray	.NET Core 3.1	1000	810.0 ns	12.69 ns	11.25 ns	8024 B
IterateAsImmutableArray	.NET 5.0	1000	769.17 ns	14.466 ns	27.171 ns	8024 B
IterateAsImmutableArray	.NET 6.0	1000	824.53 ns	18.319 ns	52.560 ns	8024 B
IterateAsImmutableArray	.NET 7.0	1000	793.43 ns	15.659 ns	22.953 ns	8024 B
IterateAsImmutableArray	.NET 8.0	1000	760.47 ns	15.204 ns	26.226 ns	8024 B

Observations

• The interface is more than just a type change, it's a behavior change
  • Iterating through IList or ICollection is > 10x slower (on .NET 6 or 7) than iterating through List
  • The reason for this is List.GetEnumerator returns a List-specific concrete class with direct method calls while IList.GetEnumerator returns a generic IEnumerator<T> with virtual method calls.
• Iterating over IEnumerable vs. ICollection is the same
• Iterating over IReadOnlyCollection vs. ICollection is the same
• Converting a List to a mutable array provides no performance benefit and duplicates the memory allocation
• Converting the List to a read-only array vs. IReadOnlyCollection (Array.AsReadOnly(data.ToArray) vs. List.AsReadOnly) is nearly 40% faster but at the expense of duplicating memory allocation
• Using ImmutableList is slow and uses 6x the memory allocation
• Iterating over IImmutableList vs. IReadOnlyCollection for ImmutableList is the same
• Converting to and using ImmutableArray is faster than converting to and using read-only array with slightly less memory overhead but only if not cast to IReadOnlyCollection!
• List iteration got much faster between .NET Core 5 and .NET 6
• .NET 8 is considerably faster in most scenarios than all prior versions
• Converting a mutable List (that never changes) to a Span first is fastest of all

Conclusions

• For public APIs: for best safety, lowest memory usage, and best performance, use IReadOnlyCollection / List.AsReadOnly.
• For private APIs: for best performance, if read-only safety is not important, keep as List, and if you can guarantee the List doesn't change, consider converting to a Span (via CollectionsMarshal.AsSpan) first
• For private APIs: for best safety and best performance, use ImmutableArray if iterating many times, or IReadOnlyCollection / List.AsReadOnly if iterating minimally
• Do not use ImmutableList for this use case (the ImmutableCollections are certainly appropriate when concurrent modification is needed)

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Aggregate list using LINQ

Another common pattern is similar to the iteration example above, but instead of simple iteration with foreach, the program performs a LINQ operation on the collection. The included LinqBenchy tests the performance and memory characteristics of various collection types when used in this pattern for both a list of 100 items and a list of 1000 items.

Benchmark Results

BenchmarkDotNet=v0.13.2, OS=macOS Monterey 12.6.1 (21G217) [Darwin 21.6.0]

Intel Core i9-9980HK CPU 2.40GHz, 1 CPU, 16 logical and 8 physical cores

.NET SDK=7.0.100

[Host] : .NET 7.0.0 (7.0.22.51805), X64 RyuJIT AVX2
Job-SYVTEW : .NET Core 3.1.20 (CoreCLR 4.700.21.47003, CoreFX 4.700.21.47101), X64 RyuJIT AVX2
Job-CLISTV : .NET 6.0.8 (6.0.822.36306), X64 RyuJIT AVX2
Job-FLHOKN : .NET 7.0.0 (7.0.22.51805), X64 RyuJIT AVX2

Method	Runtime	N	Mean	Error	StdDev	Allocated
AggregateAsMutableList	.NET Core 3.1	100	755.2 ns	6.13 ns	5.74 ns	40 B
AggregateAsMutableList	.NET 6.0	100	770.7 ns	6.29 ns	5.88 ns	40 B
AggregateAsMutableList	.NET 7.0	100	796.4 ns	13.69 ns	12.80 ns	40 B
AggregateAsMutableIList	.NET Core 3.1	100	755.4 ns	6.27 ns	5.56 ns	40 B
AggregateAsMutableIList	.NET 6.0	100	769.2 ns	5.75 ns	5.38 ns	40 B
AggregateAsMutableIList	.NET 7.0	100	787.3 ns	7.13 ns	5.95 ns	40 B
AggregateAsMutableICollection	.NET Core 3.1	100	755.1 ns	9.67 ns	9.04 ns	40 B
AggregateAsMutableICollection	.NET 6.0	100	767.5 ns	8.16 ns	7.64 ns	40 B
AggregateAsMutableICollection	.NET 7.0	100	786.4 ns	6.86 ns	6.41 ns	40 B
AggregateAsIEnumerable	.NET Core 3.1	100	752.4 ns	5.58 ns	4.66 ns	40 B
AggregateAsIEnumerable	.NET 6.0	100	765.5 ns	5.42 ns	5.07 ns	40 B
AggregateAsIEnumerable	.NET 7.0	100	783.9 ns	6.26 ns	5.55 ns	40 B
AggregateAsMutableArray	.NET Core 3.1	100	614.4 ns	8.02 ns	7.50 ns	856 B
AggregateAsMutableArray	.NET 6.0	100	618.0 ns	5.68 ns	5.32 ns	856 B
AggregateAsMutableArray	.NET 7.0	100	591.7 ns	9.14 ns	8.10 ns	856 B
AggregateAsIReadOnlyCollection	.NET Core 3.1	100	810.2 ns	15.17 ns	13.44 ns	64 B
AggregateAsIReadOnlyCollection	.NET 6.0	100	733.8 ns	7.98 ns	7.46 ns	64 B
AggregateAsIReadOnlyCollection	.NET 7.0	100	781.8 ns	12.64 ns	11.20 ns	64 B
AggregateAsReadOnlyArray	.NET Core 3.1	100	580.6 ns	5.73 ns	4.79 ns	880 B
AggregateAsReadOnlyArray	.NET 6.0	100	586.4 ns	9.12 ns	8.53 ns	880 B
AggregateAsReadOnlyArray	.NET 7.0	100	597.1 ns	8.13 ns	7.61 ns	880 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET Core 3.1	100	5,244.2 ns	46.73 ns	39.02 ns	4920 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET 6.0	100	4,121.0 ns	33.36 ns	26.04 ns	4920 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET 7.0	100	4,189.0 ns	28.81 ns	24.06 ns	4920 B
AggregateAsIImmutableList	.NET Core 3.1	100	5,238.3 ns	54.66 ns	51.13 ns	4920 B
AggregateAsIImmutableList	.NET 6.0	100	4,155.4 ns	31.51 ns	27.93 ns	4920 B
AggregateAsIImmutableList	.NET 7.0	100	4,207.5 ns	34.46 ns	28.78 ns	4920 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET Core 3.1	100	650.1 ns	7.28 ns	6.81 ns	880 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET 6.0	100	668.7 ns	9.49 ns	8.88 ns	880 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET 7.0	100	641.6 ns	11.48 ns	10.18 ns	880 B
AggregateAsImmutableArray	.NET Core 3.1	100	256.5 ns	3.16 ns	2.64 ns	824 B
AggregateAsImmutableArray	.NET 6.0	100	260.1 ns	3.45 ns	3.06 ns	824 B
AggregateAsImmutableArray	.NET 7.0	100	248.8 ns	3.06 ns	2.71 ns	824 B
AggregateAsMutableList	.NET Core 3.1	1000	7,060.8 ns	44.88 ns	39.79 ns	40 B
AggregateAsMutableList	.NET 6.0	1000	7,559.2 ns	148.54 ns	193.14 ns	40 B
AggregateAsMutableList	.NET 7.0	1000	7,686.5 ns	150.88 ns	148.19 ns	40 B
AggregateAsMutableIList	.NET Core 3.1	1000	7,165.6 ns	103.28 ns	96.61 ns	40 B
AggregateAsMutableIList	.NET 6.0	1000	7,438.7 ns	68.86 ns	61.05 ns	40 B
AggregateAsMutableIList	.NET 7.0	1000	7,654.0 ns	69.92 ns	61.98 ns	40 B
AggregateAsMutableICollection	.NET Core 3.1	1000	7,503.1 ns	101.75 ns	95.18 ns	40 B
AggregateAsMutableICollection	.NET 6.0	1000	7,481.6 ns	96.30 ns	85.37 ns	40 B
AggregateAsMutableICollection	.NET 7.0	1000	7,641.9 ns	65.00 ns	50.74 ns	40 B
AggregateAsIEnumerable	.NET Core 3.1	1000	7,435.7 ns	63.33 ns	59.24 ns	40 B
AggregateAsIEnumerable	.NET 6.0	1000	7,460.6 ns	98.28 ns	91.93 ns	40 B
AggregateAsIEnumerable	.NET 7.0	1000	7,664.0 ns	70.63 ns	66.07 ns	40 B
AggregateAsMutableArray	.NET Core 3.1	1000	5,865.3 ns	77.47 ns	64.69 ns	8056 B
AggregateAsMutableArray	.NET 6.0	1000	5,940.8 ns	66.55 ns	62.25 ns	8056 B
AggregateAsMutableArray	.NET 7.0	1000	5,643.5 ns	65.46 ns	58.03 ns	8056 B
AggregateAsIReadOnlyCollection	.NET Core 3.1	1000	7,811.7 ns	144.69 ns	128.26 ns	64 B
AggregateAsIReadOnlyCollection	.NET 6.0	1000	6,950.4 ns	44.28 ns	34.57 ns	64 B
AggregateAsIReadOnlyCollection	.NET 7.0	1000	7,890.4 ns	46.26 ns	38.63 ns	64 B
AggregateAsReadOnlyArray	.NET Core 3.1	1000	5,415.1 ns	39.51 ns	32.99 ns	8080 B
AggregateAsReadOnlyArray	.NET 6.0	1000	5,963.2 ns	111.81 ns	104.59 ns	8080 B
AggregateAsReadOnlyArray	.NET 7.0	1000	5,441.7 ns	72.09 ns	67.43 ns	8080 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET Core 3.1	1000	52,002.4 ns	584.07 ns	546.34 ns	48120 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET 6.0	1000	39,917.2 ns	592.72 ns	525.43 ns	48120 B
AggregateAsImmutableListCastToIReadOnlyCollection	.NET 7.0	1000	40,370.9 ns	526.35 ns	492.34 ns	48120 B
AggregateAsIImmutableList	.NET Core 3.1	1000	51,796.8 ns	606.88 ns	506.77 ns	48120 B
AggregateAsIImmutableList	.NET 6.0	1000	39,648.7 ns	162.09 ns	126.55 ns	48120 B
AggregateAsIImmutableList	.NET 7.0	1000	40,401.5 ns	570.26 ns	533.42 ns	48120 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET Core 3.1	1000	5,775.7 ns	51.30 ns	47.98 ns	8080 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET 6.0	1000	6,248.6 ns	120.75 ns	129.21 ns	8080 B
AggregateAsImmutableArrayCastToIReadOnlyCollection	.NET 7.0	1000	5,715.6 ns	61.33 ns	57.36 ns	8080 B
AggregateAsImmutableArray	.NET Core 3.1	1000	2,205.1 ns	40.74 ns	34.02 ns	8024 B
AggregateAsImmutableArray	.NET 6.0	1000	2,256.3 ns	33.17 ns	27.70 ns	8024 B
AggregateAsImmutableArray	.NET 7.0	1000	2,238.4 ns	19.07 ns	17.84 ns	8024 B

Observations

• The difference that was present in the Iteration benchmark between interface and implementation types isn't there; the implementation type has no speed advantage over the interface type
• ImmutableList is still the worst
• ImmutableArray is the best performance of all (as long as not cast to IReadOnlyCollection)

Conclusions

The same as in the Iteration benchmark, except here there's no penalty for defining your APIs with IReadOnlyCollection / ICollection / IEnumerable if keeping the collection as a List. Also, prefer ToImmutableArray over Array.AsReadOnly(data.ToArray()) as both memory consumption and performance is better.