Why is this an issue?

Both the List.Exists method and IEnumerable.Any method can be used to find the first element that satisfies a predicate in a collection. However, List.Exists can be faster than IEnumerable.Any for List objects, as well as requires significantly less memory. For small collections, the performance difference may be negligible, but for large collections, it can be noticeable. The same applies to ImmutableList and arrays too.

Applies to

• List
• Array
• ImmutableList

What is the potential impact?

We measured at least 3x improvement in execution time. For more details see the Benchmarks section from the More info tab.

Also, no memory allocations were needed for the Exists method, since the search is done in-place.

Exceptions

Since LINQ to Entities relies a lot on System.Linq for query conversion, this rule won’t raise when used within LINQ to Entities syntaxes.

How to fix it

The Exists method is defined on the collection class, and it has the same signature as Any extension method if a predicate is used. The method can be replaced in place.

Code examples

Noncompliant code example

Function ContainsEven(data As List(Of Integer)) As Boolean
    Return data.Any(Function(x) x Mod 2 = 0)
End Function

Function ContainsEven(data() As Integer) As Boolean
    Return data.Any(Function(x) x Mod 2 = 0)
End Function

Compliant solution

Function ContainsEven(data As List(Of Integer)) As Boolean
    Return data.Exists(Function(x) x Mod 2 = 0)
End Function

Function ContainsEven(data() As Integer) As Boolean
    Return Array.Exists(data, Function(x) x Mod 2 = 0)
End Function

Resources

Documentation

• List<T>.Exists(Predicate<T>)
• Array.Exists<T>(T[], Predicate<T>)
• ImmutableList<T>.Exists(Predicate<T>)
• Enumerable.Any(Predicate<T>)
• LINQ to Entities

Benchmarks

The results were generated by running the following snippet with BenchmarkDotNet:

private List<int> data;
private readonly Random random = new Random();

[Params(1_000)]
public int N { get; set; }

[GlobalSetup]
public void Setup() =>
    data = Enumerable.Range(0, N).Select(x => 43).ToList();

[Benchmark(Baseline = true)]
public void Any()
{
    for (var i = 0; i < N; i++)
    {
        _ = data.Any(x => x % 2 == 0);          // Enumerable.Any
    }
}

[Benchmark]
public void Exists()
{
    for (var i = 0; i < N; i++)
    {
        _ = data.Exists(x => x % 2 == 0);       // List<T>.Exists
    }
}

Hardware configuration:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2846/22H2/2022Update)
11th Gen Intel Core i7-11850H 2.50GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK=7.0.203
  [Host]               : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  .NET 7.0             : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  .NET Framework 4.6.2 : .NET Framework 4.8.1 (4.8.9139.0), X64 RyuJIT VectorSize=256