HLQ005: Avoid use of Single(), SingleOrDefault(), SingleAsync() and SingleOrDefaultAsync() operations

Cause

Methods Single(), SingleOrDefault(), SingleAsync() or SingleOrDefaultAsync() are used to get the first element of a collection.

Severity

Warning

Rule description

The methods Single(), SingleOrDefault(), First() and FirstOrDefault() (plus the async counterparts) are typically used get the first element of a LINQ query. Verifying if the result of the query contains a single item requires applying the query to the souce items until a second item is found or, until the end of the collection. This can be a very expensive operation.

If the query returns more than one item and it's not expected, there is either a problem with the query of in the source collection. Verifying these conditions at runtime is not a good practice.

Benchmarks

Comparing the performance of Single() and First() for the best case scenario, where the first element of the collection satisfies the query conditions, and the worst case, where only the last element of the collection does not satisfy the query conditions.

Source: https://github.com/NetFabric/NetFabric.Hyperlinq.Analyzer/blob/master/NetFabric.Hyperlinq.Analyzer.Benchmarks/HLQ005_AvoidSingleAnalyzer.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	Categories	Count	Mean	Error	StdDev	Median	Ratio	RatioSD	Gen0	Code Size	Allocated	Alloc Ratio
BestCase_Single	.NET 6	.NET 6.0	BestCase	100	620.48 ns	10.790 ns	13.646 ns	616.74 ns	baseline	****	0.0153	547 B	32 B	****
BestCase_First	.NET 6	.NET 6.0	BestCase	100	17.92 ns	0.197 ns	0.242 ns	17.94 ns	34.66x faster	0.72x	0.0153	456 B	32 B	1.00x more
BestCase_Single	.NET 7	.NET 7.0	BestCase	100	647.87 ns	7.507 ns	5.861 ns	649.43 ns	baseline		0.0153	510 B	32 B
BestCase_First	.NET 7	.NET 7.0	BestCase	100	19.52 ns	0.371 ns	0.329 ns	19.51 ns	33.19x faster	0.69x	0.0153	430 B	32 B	1.00x more
BestCase_Single	.NET 8	.NET 8.0	BestCase	100	287.77 ns	3.058 ns	2.554 ns	287.55 ns	baseline		0.0153	1,036 B	32 B
BestCase_First	.NET 8	.NET 8.0	BestCase	100	13.98 ns	0.145 ns	0.129 ns	13.97 ns	20.57x faster	0.20x	0.0153	659 B	32 B	1.00x more
BestCase_Single	.NET 6	.NET 6.0	BestCase	10000	59,386.15 ns	428.718 ns	401.023 ns	59,392.95 ns	baseline	****	-	547 B	32 B	****
BestCase_First	.NET 6	.NET 6.0	BestCase	10000	18.41 ns	0.272 ns	0.213 ns	18.34 ns	3,225.926x faster	49.88x	0.0153	456 B	32 B	1.00x more
BestCase_Single	.NET 7	.NET 7.0	BestCase	10000	59,037.31 ns	333.439 ns	260.327 ns	59,069.24 ns	baseline		-	510 B	32 B
BestCase_First	.NET 7	.NET 7.0	BestCase	10000	20.71 ns	0.955 ns	2.710 ns	19.22 ns	2,894.540x faster	284.74x	0.0153	430 B	32 B	1.00x more
BestCase_Single	.NET 8	.NET 8.0	BestCase	10000	31,992.80 ns	280.590 ns	219.066 ns	31,943.08 ns	baseline		-	883 B	32 B
BestCase_First	.NET 8	.NET 8.0	BestCase	10000	13.83 ns	0.308 ns	0.400 ns	13.75 ns	2,332.087x faster	61.31x	0.0153	660 B	32 B	1.00x more
WorstCase_Single	.NET 6	.NET 6.0	WorstCase	100	603.35 ns	9.153 ns	7.643 ns	601.27 ns	baseline	****	0.0153	547 B	32 B	****
WorstCase_First	.NET 6	.NET 6.0	WorstCase	100	620.98 ns	6.142 ns	5.129 ns	620.11 ns	1.03x slower	0.02x	0.0153	456 B	32 B	1.00x more
WorstCase_Single	.NET 7	.NET 7.0	WorstCase	100	644.13 ns	5.286 ns	5.875 ns	642.98 ns	baseline		0.0153	510 B	32 B
WorstCase_First	.NET 7	.NET 7.0	WorstCase	100	646.30 ns	10.952 ns	8.550 ns	644.88 ns	1.00x slower	0.02x	0.0153	430 B	32 B	1.00x more
WorstCase_Single	.NET 8	.NET 8.0	WorstCase	100	287.32 ns	2.771 ns	2.457 ns	286.77 ns	baseline		0.0153	826 B	32 B
WorstCase_First	.NET 8	.NET 8.0	WorstCase	100	307.45 ns	3.885 ns	4.914 ns	306.72 ns	1.07x slower	0.02x	0.0153	639 B	32 B	1.00x more
WorstCase_Single	.NET 6	.NET 6.0	WorstCase	10000	57,425.36 ns	1,099.017 ns	1,079.382 ns	57,099.79 ns	baseline	****	-	547 B	32 B	****
WorstCase_First	.NET 6	.NET 6.0	WorstCase	10000	60,242.67 ns	1,124.441 ns	1,998.693 ns	59,291.72 ns	1.05x slower	0.04x	-	456 B	32 B	1.00x more
WorstCase_Single	.NET 7	.NET 7.0	WorstCase	10000	61,896.58 ns	647.584 ns	636.014 ns	61,721.18 ns	baseline		-	510 B	32 B
WorstCase_First	.NET 7	.NET 7.0	WorstCase	10000	62,517.62 ns	945.663 ns	1,776.185 ns	61,873.28 ns	1.02x slower	0.04x	-	430 B	32 B	1.00x more
WorstCase_Single	.NET 8	.NET 8.0	WorstCase	10000	26,906.21 ns	331.350 ns	309.945 ns	26,867.91 ns	baseline		-	765 B	32 B
WorstCase_First	.NET 8	.NET 8.0	WorstCase	10000	30,013.80 ns	593.585 ns	1,252.072 ns	29,412.80 ns	1.12x slower	0.05x	-	594 B	32 B	1.00x more

How to fix violations

Use First(), FirstOrDefault(), FirstAsync() or FirstOrDefaultAsync() methods to get the first element of a query.

When to suppress warnings

Suppress when unit testing, or validating data, and want to guarantee that the collection does not contain duplicates.

Example of a violation

public static Employee GetEmployee(this IEnumerable<Employee> employees, int employeeId)
    => employees.SingleOrDefault(employee => employee.Id == employeeId);

Example of how to fix

public static Employee GetEmployee(this IEnumerable<Employee> employees, int employeeId)
    => employees.FirstOrDefault(employee => employee.Id == employeeId);