UniqueNonSetCollection

Diagnostic Rule Overview

Field	Value
ID	SHIMMER1102
Analyzer title	Use a set instead
Analyzer message	Prefer sets when uniqueness is required
Code fix title	Replace .Distinct().ToList() or .Distinct().ToArray() with .ToHashSet()
Default severity	Info
Minimum framework/language version	.NET Core 2.0+, .NET Framework 4.7.2+, .NET Standard 2.1
Enabled by default?	No
Category	ShimmeringUsage
Link to code	UniqueNonSetCollectionAnalyzer.cs
Code fix exists?	Yes

Detailed Explanation

Using .Distinct().ToArray() or .Distinct().ToList() suggests that the goal is to remove duplicates. A HashSet<T> is a data structure that may better fit your use case, as it achieves the same outcome more efficiently while allowing a faster lookup.

Examples

Flagged code:

using System.Collections.Generic;
using System.Linq;

namespace Tests;
class Test
{
    void Do()
    {
        List<int> numbers = [];
        var distinctNumbers = numbers.Distinct().ToArray();
    }
}

Fixed code:

using System.Collections.Generic;
using System.Linq;

namespace Tests;
class Test
{
    void Do()
    {
        List<int> numbers = [];
        var distinctNumbers = numbers.ToHashSet();
    }
}

Justification of the Severity

This diagnostic is disabled by default because this diagnostic can cause compilation errors, which can be disruptive.

When to Suppress

Suppress this diagnostic if changing the type breaks an existing contract or you need the collection to be ordered or indexed.

Inspiration

Suggested by @matthew-elgart.