HLQ012: Use CollectionsMarshal.AsSpan() to iterate List.

Cause

'foreach' loop is being used on a List<T>.

Severity

Warning

Rule description

When iterating over a List<T> in a foreach loop, the default behavior is to create an enumerator and allocate an iterator object. This incurs additional memory allocations and can impact performance, especially in scenarios with large collections or frequent iterations. By using the CollectionsMarshal.AsSpan() method, you can eliminate these overheads and directly access the underlying elements of the List<T> without creating an enumerator. Iterating a Span<T> directly avoids the overhead of the enumerator, resulting in faster loop iterations.

Benchmarking

Source: https://github.com/NetFabric/NetFabric.Hyperlinq.Analyzer/blob/master/NetFabric.Hyperlinq.Analyzer.Benchmarks/HLQ012_UseCollectionsMarshalAsSpanAnalyzer.cs

BenchmarkDotNet v0.13.6, Windows 10 (10.0.19045.3155/22H2/2022Update)
Intel Core i7-7567U CPU 3.50GHz (Kaby Lake), 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100-preview.5.23303.2
  [Host] : .NET 6.0.20 (6.0.2023.32017), X64 RyuJIT AVX2
  .NET 6 : .NET 6.0.20 (6.0.2023.32017), X64 RyuJIT AVX2
  .NET 7 : .NET 7.0.8 (7.0.823.31807), X64 RyuJIT AVX2
  .NET 8 : .NET 8.0.0 (8.0.23.28008), X64 RyuJIT AVX2

Method	Job	Runtime	Count	Mean	Error	StdDev	Median	Ratio	RatioSD	Code Size	Allocated	Alloc Ratio
ForEach	.NET 6	.NET 6.0	0	1.0956 ns	0.0530 ns	0.1300 ns	1.0414 ns	baseline	****	99 B	-	NA
CollectionsMarshalAsSpan	.NET 6	.NET 6.0	0	0.5871 ns	0.0329 ns	0.0379 ns	0.5835 ns	1.83x faster	0.16x	NA	-	NA
ForEach	.NET 7	.NET 7.0	0	1.3218 ns	0.0395 ns	0.0440 ns	1.3141 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 7	.NET 7.0	0	0.8831 ns	0.0532 ns	0.1510 ns	0.8114 ns	1.50x faster	0.20x	NA	-	NA
ForEach	.NET 8	.NET 8.0	0	0.0441 ns	0.0310 ns	0.0793 ns	0.0000 ns	?	?	NA	-	?
CollectionsMarshalAsSpan	.NET 8	.NET 8.0	0	0.4798 ns	0.0416 ns	0.1146 ns	0.4198 ns	?	?	NA	-	?
ForEach	.NET 6	.NET 6.0	10	14.2010 ns	0.1204 ns	0.1005 ns	14.1930 ns	baseline	****	99 B	-	NA
CollectionsMarshalAsSpan	.NET 6	.NET 6.0	10	4.0248 ns	0.1112 ns	0.1189 ns	3.9801 ns	3.51x faster	0.11x	NA	-	NA
ForEach	.NET 7	.NET 7.0	10	10.3653 ns	0.1658 ns	0.1470 ns	10.3078 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 7	.NET 7.0	10	3.6369 ns	0.1270 ns	0.3583 ns	3.4921 ns	2.92x faster	0.21x	NA	-	NA
ForEach	.NET 8	.NET 8.0	10	7.1217 ns	0.1269 ns	0.1125 ns	7.0938 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 8	.NET 8.0	10	3.8439 ns	0.1792 ns	0.5084 ns	3.6169 ns	1.88x faster	0.24x	NA	-	NA
ForEach	.NET 6	.NET 6.0	100	142.4614 ns	2.6891 ns	3.4966 ns	142.0638 ns	baseline	****	99 B	-	NA
CollectionsMarshalAsSpan	.NET 6	.NET 6.0	100	46.1275 ns	0.9312 ns	0.9963 ns	46.0060 ns	3.09x faster	0.12x	NA	-	NA
ForEach	.NET 7	.NET 7.0	100	113.1277 ns	2.2898 ns	2.9773 ns	111.7294 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 7	.NET 7.0	100	37.8990 ns	0.5435 ns	0.5816 ns	37.6491 ns	3.00x faster	0.10x	NA	-	NA
ForEach	.NET 8	.NET 8.0	100	67.2375 ns	1.3082 ns	1.1596 ns	66.9356 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 8	.NET 8.0	100	39.8413 ns	0.4387 ns	0.4308 ns	39.7005 ns	1.69x faster	0.04x	NA	-	NA
ForEach	.NET 6	.NET 6.0	1000	1,316.1551 ns	8.4594 ns	6.6045 ns	1,315.7101 ns	baseline	****	99 B	-	NA
CollectionsMarshalAsSpan	.NET 6	.NET 6.0	1000	392.1319 ns	6.4124 ns	6.8612 ns	388.7779 ns	3.34x faster	0.07x	NA	-	NA
ForEach	.NET 7	.NET 7.0	1000	1,045.3751 ns	5.2175 ns	4.3569 ns	1,045.0901 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 7	.NET 7.0	1000	325.3411 ns	2.4156 ns	2.0171 ns	325.4291 ns	3.21x faster	0.03x	NA	-	NA
ForEach	.NET 8	.NET 8.0	1000	594.4719 ns	7.7370 ns	6.0406 ns	593.9274 ns	baseline		NA	-	NA
CollectionsMarshalAsSpan	.NET 8	.NET 8.0	1000	352.6162 ns	5.7492 ns	10.3669 ns	348.4033 ns	1.67x faster	0.06x	NA	-	NA

How to fix violations

Use the CollectionsMarshal.AsSpan() method to convert List<T> to a Span<T>.

When to suppress warnings

The CollectionsMarshal.AsSpan() method is only available in .NET Standard 2.1 or later, and .NET Core 3.0 or later. Make sure your project targets a compatible framework version.

It's important to validate that using CollectionsMarshal.AsSpan() is suitable for your specific scenario. While it provides performance benefits, it may not be appropriate for all use cases.

Example of a violation

var source = new List<int>();
foreach (var item in source)
    Console.WriteLine(item);

Example of how to fix

var source = new List<int>();
foreach (var item in CollectionsMarshal.AsSpan(source))
    Console.WriteLine(item);