SequenceEqual is true but HashSet.SetEquals is false

Question

I've been experimenting with implementing Set equality (ie List comparisons where the order is irrelevant) and after reading SO questions like this and this , wrote the following simple extension:

    public static bool SetEqual<T>(this IEnumerable<T> enumerable, IEnumerable<T> other)
    {
        if (enumerable == null && other == null)
            return true;

        if (enumerable == null || other == null)
            return false;

        var setA = new HashSet<T>(enumerable);
        return setA.SetEquals(other);
    }

However, I came across a simple data structure for which this approach does not work, while Enumerable.SequenceEqual does.

    public class Test : IEquatable<Test>
    {
        public Guid Id { get; set; }
        public List<Test> RelatedTest { get; set; }

        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null, obj)) return false;
            if (ReferenceEquals(this, obj)) return true;
            if (obj.GetType() != typeof(Test)) return false;

            return Equals((Test)obj);
        }

        public bool Equals(Test other)
        {
            if (ReferenceEquals(null, other)) return false;
            if (ReferenceEquals(this, other)) return true;

            return other.Id.Equals(Id) &&
                   RelatedTest.SetEqual(other.RelatedTest);
        }
    }

Given this object, this test succeeds:

    [Test]
    public void SequenceEqualTest()
    {
        var test1 = new List<Test> {new Test()};
        var test2 = new List<Test> {new Test() };

        Assert.That(test1.SequenceEqual(test2), Is.True);
    }

But this test fails:

    [Test]
    public void SetEqualTest()
    {
        var test1 = new List<Test> {new Test()};
        var test2 = new List<Test> {new Test()};

        Assert.That(test1.SetEqual(test2), Is.True);
    }

Does anyone have an explanation?

Answer 1

Yes, you didn't override GetHashCode in your Test class, so the HashSet is unable to effectively group items into buckets according to possible equality. See this question for more details: Why is it important to override GetHashCode when Equals method is overridden?