HLQ013: Use foreach loop

July 26, 2023 · View on GitHub

Cause

for loop is being used to iterate an array or a span.

Both for and foreach loops can be used to iterate an array or a span. The code generated by the compiler for both is actually very similar. The difference is that the code generated for the foreach guarantees the following:

The collection is a local variable.
The terminal condition is the length of the collection.

These allow the JIT compiler to drop bounds checking making the iteration more performant.

Benchmarks

Source: https://github.com/NetFabric/NetFabric.Hyperlinq.Analyzer/blob/master/NetFabric.Hyperlinq.Analyzer.Benchmarks/HLQ013_UseForEachLoop.cs


BenchmarkDotNet v0.13.6, Windows 10 (10.0.19045.3269/22H2/2022Update)
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
For	.NET 6	.NET 6.0	10	4.811 ns	0.1636 ns	0.4693 ns	4.649 ns	baseline	****	67 B	-	NA
ForEach	.NET 6	.NET 6.0	10	3.828 ns	0.1716 ns	0.4923 ns	3.726 ns	1.28x faster	0.20x	NA	-	NA
ForEach_Span	.NET 6	.NET 6.0	10	4.460 ns	0.1720 ns	0.4936 ns	4.311 ns	1.09x faster	0.15x	NA	-	NA

For	.NET 7	.NET 7.0	10	4.728 ns	0.1921 ns	0.5634 ns	4.487 ns	baseline		67 B	-	NA
ForEach	.NET 7	.NET 7.0	10	3.322 ns	0.1566 ns	0.4468 ns	3.211 ns	1.43x faster	0.23x	NA	-	NA
ForEach_Span	.NET 7	.NET 7.0	10	3.728 ns	0.2446 ns	0.6979 ns	3.438 ns	1.29x faster	0.25x	NA	-	NA

For	.NET 8	.NET 8.0	10	4.612 ns	0.1770 ns	0.4905 ns	4.450 ns	baseline		67 B	-	NA
ForEach	.NET 8	.NET 8.0	10	4.065 ns	0.3462 ns	0.9932 ns	3.608 ns	1.19x faster	0.25x	NA	-	NA
ForEach_Span	.NET 8	.NET 8.0	10	4.347 ns	0.1496 ns	0.4268 ns	4.145 ns	1.07x faster	0.14x	NA	-	NA

For	.NET 6	.NET 6.0	1000	623.195 ns	11.9510 ns	9.9796 ns	623.367 ns	baseline	****	67 B	-	NA
ForEach	.NET 6	.NET 6.0	1000	452.951 ns	9.0306 ns	25.1737 ns	449.691 ns	1.36x faster	0.07x	NA	-	NA
ForEach_Span	.NET 6	.NET 6.0	1000	437.701 ns	7.5122 ns	13.7366 ns	433.143 ns	1.42x faster	0.04x	NA	-	NA

For	.NET 7	.NET 7.0	1000	488.039 ns	4.7154 ns	4.1801 ns	487.612 ns	baseline		67 B	-	NA
ForEach	.NET 7	.NET 7.0	1000	392.328 ns	5.3285 ns	4.7236 ns	391.128 ns	1.24x faster	0.02x	NA	-	NA
ForEach_Span	.NET 7	.NET 7.0	1000	405.471 ns	7.7527 ns	12.0700 ns	399.877 ns	1.19x faster	0.05x	NA	-	NA

For	.NET 8	.NET 8.0	1000	492.753 ns	8.9344 ns	10.6358 ns	489.309 ns	baseline		67 B	-	NA
ForEach	.NET 8	.NET 8.0	1000	414.904 ns	9.6482 ns	27.8373 ns	402.107 ns	1.20x faster	0.07x	NA	-	NA
ForEach_Span	.NET 8	.NET 8.0	1000	414.874 ns	9.7770 ns	28.2089 ns	406.918 ns	1.19x faster	0.09x	NA	-	NA

How to fix violations

Use foreach instead of for when iterating over an array or a span.

For partial iterations, use a range-based foreach loop.

When to suppress warnings

When the indexing is required for some reason, e.g. when the index is used to access another array.

Example of a violation

Full iteration of an array using a for loop:

var source = new[] { 1, 2, 3 };
for (var index = 0; index < source.Length; index++)
{
    var item = source[index];
    Console.WriteLine(item);
}

Partial iteration of an array using a for loop:


``` csharp
var source = new[] { 1, 2, 3 };
// skiping first and last items
for (var index = 1; index < source.Length - 1; index++)
{
    var item = source[index];
    Console.WriteLine(item);
}

Example of how to fix

Full iteration of an array using a foreach loop:

var source = new[] { 1, 2, 3 };
foreach (var item in source)
{
    Console.WriteLine(item);
}

Partial iteration of an array using a foreach loop:

var source = new[] { 1, 2, 3 };
// skiping first and last items
foreach (var item in source[1..^1])
{
    Console.WriteLine(item);
}