xUnit2026 Warning

Comparison of sets must be done with IEqualityComparer

Cause

This rule is triggered by the use of an equality assertion with two sets, using an item comparison function.

Reason for rule

Unlike linear containers (arrays, lists, etc.), sets do not have a natural order to them. A simple item comparison function is not sufficient, due to the way sets define item equality.

For more information, see Equality with hash sets vs. linear containers.

How to fix violations

In order for set item comparisons to perform properly, they need both an implementation of Equals and an implementation of GetHashCode. More importantly, any two values which return true for Equals must also return the same value for GetHashCode. Because both of these functions need to be defined, and because the implementation of GetHashCode cannot be inferred from the equality function, you are required to provide both pieces of code. This is fundamental to how sets work.

If the item in the set is a custom class that you control, then you can override the implementation of both Equals and GetHashCode on the custom class. These functions will be used by the set when you don’t have a custom item comparer. This is the recommended way to do this when you control the definition of the item type.

If you don’t control the definition of the item being placed into the set, then you need to implement a custom IEqualityComparer<T> for the type in question. Once you have the custom comparer, there are two ways you can use it. The preferred way is to pass it to the set during construction so that it will be used for all comparison operations. Doing this means you don’t have to pass the comparer to the assertion function.

If you don’t control the creation of the set, the final way to make this work is to pass the custom item comparer to the assertion function. Note that in this case, the assertion library must make new copies of the expected and actual sets using the new comparer, and copy all the items into the new sets. This means the assertion will be slower and use more memory (how much slower and how much more memory depends on the size of the sets in question, among other variables).

Important debugging note: If your tests are failing when you think they should be passing, please double check your GetHashCode implementation to ensure equal values have equal hash codes. In the samples below, where our equality test is case-insensitive, we make sure to pass uppercase normalized values into the hash code combiner function; if we just passed the original values, then Brad and BRAD would have different hash code values, so the set would not consider them to be equal. Sets always consult hash codes before equality functions to determine item equality.

Examples

Violates

using System;
using System.Collections.Generic;
using Xunit;

public class Person(string firstName, string lastName)
{
    public string FirstName { get; } = firstName;
    public string LastName { get; } = lastName;
}

public class xUnit2026
{
    [Fact]
    public void TestMethod()
    {
        var set1 = new HashSet<Person> { new("Brad", "Wilson") };
        var set2 = new HashSet<Person> { new("BRAD", "WILSON") };

        Assert.Equal(
            set1, set2,
            (Person x, Person y) =>
                x.FirstName.Equals(y.FirstName, StringComparison.CurrentCultureIgnoreCase)
                && x.LastName.Equals(y.LastName, StringComparison.CurrentCultureIgnoreCase)
        );
    }
}

Does not violate

Override `Equals` and `GetHashCode` on the item

using System;
using System.Collections.Generic;
using System.Globalization;
using Xunit;

public class Person(string firstName, string lastName)
{
    public string FirstName { get; } = firstName;
    public string LastName { get; } = lastName;

    public override bool Equals(object? obj)
    {
        if (obj is not Person other)
            return false;

        return FirstName.Equals(other.FirstName, StringComparison.CurrentCultureIgnoreCase)
            && LastName.Equals(other.LastName, StringComparison.CurrentCultureIgnoreCase);
    }

    public override int GetHashCode() =>
        HashCode.Combine(
            FirstName.ToUpper(CultureInfo.CurrentCulture),
            LastName.ToUpper(CultureInfo.CurrentCulture)
        );
}

public class xUnit2026
{
    [Fact]
    public void TestMethod()
    {
        var set1 = new HashSet<Person> { new("Brad", "Wilson") };
        var set2 = new HashSet<Person> { new("BRAD", "WILSON") };

        Assert.Equal(set1, set2);
    }
}

Pass the comparer to the set during construction

using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using Xunit;

public class Person(string firstName, string lastName)
{
    public string FirstName { get; } = firstName;
    public string LastName { get; } = lastName;
}

public class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person? x, Person? y)
    {
        if (x is null)
            return y is null;
        if (y is null)
            return false;
        return x.FirstName.Equals(y.FirstName, StringComparison.CurrentCultureIgnoreCase)
            && x.LastName.Equals(y.LastName, StringComparison.CurrentCultureIgnoreCase);
    }

    public int GetHashCode([DisallowNull] Person obj) =>
        HashCode.Combine(
            obj.FirstName.ToUpper(CultureInfo.CurrentCulture),
            obj.LastName.ToUpper(CultureInfo.CurrentCulture)
        );
}

public class xUnit2026
{
    [Fact]
    public void TestMethod()
    {
        var comparer = new PersonComparer();
        var set1 = new HashSet<Person>(comparer) { new("Brad", "Wilson") };
        var set2 = new HashSet<Person>(comparer) { new("BRAD", "WILSON") };

        Assert.Equal(set1, set2);
    }
}

Pass the comparer to the assertion

using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using Xunit;

public class Person(string firstName, string lastName)
{
    public string FirstName { get; } = firstName;
    public string LastName { get; } = lastName;
}

public class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person? x, Person? y)
    {
        if (x is null)
            return y is null;
        if (y is null)
            return false;
        return x.FirstName.Equals(y.FirstName, StringComparison.CurrentCultureIgnoreCase)
            && x.LastName.Equals(y.LastName, StringComparison.CurrentCultureIgnoreCase);
    }

    public int GetHashCode([DisallowNull] Person obj) =>
        HashCode.Combine(
            obj.FirstName.ToUpper(CultureInfo.CurrentCulture),
            obj.LastName.ToUpper(CultureInfo.CurrentCulture)
        );
}

public class xUnit2026
{
    [Fact]
    public void TestMethod()
    {
        var comparer = new PersonComparer();
        var set1 = new HashSet<Person> { new("Brad", "Wilson") };
        var set2 = new HashSet<Person> { new("BRAD", "WILSON") };

        Assert.Equal(set1, set2, comparer);
    }
}