BeOneOf for object comparisons with custom comparer support

Background and motivation

Support for custom comparer to compare objects

API Proposal

public class ObjectAssertions<TSubject, TAssertions> : ReferenceTypeAssertions<TSubject, TAssertions>
    where TAssertions : ObjectAssertions<TSubject, TAssertions>
{
    public AndConstraint<TAssertions> BeOneOf<TExpectation>(IEnumerable<TExpectation> validValues, IEqualityComparer<TExpectation> comparer, string because = "", params object[] becauseArgs);
}

API Usage

var obj = new SomeClass(1);
var expected = new[] { new SomeClass(1), new SomeClass(2), new SomeClass(3) };
obj .Should().BeOneOf(expected, new SomeClassEqualityComparer ());

internal class SomeClass
{
    public SomeClass(int key)
    {
        Key = key;
    }

    public int Key { get; }

    public override string ToString()
    {
        return $"SomeClass({Key})";
    }
}

internal class SomeClassEqualityComparer : IEqualityComparer<SomeClass>
{
    public bool Equals(SomeClass x, SomeClass y)
    {
        if (ReferenceEquals(x, y))
        {
            return true;
        }

        if (ReferenceEquals(x, null))
        {
            return false;
        }

        if (ReferenceEquals(y, null))
        {
            return false;
        }

        if (x.GetType() != y.GetType())
        {
            return false;
        }

        return x.Key == y.Key;
    }

    public int GetHashCode(SomeClass obj)
    {
        return obj.Key;
    }
}

Alternative Designs

No response

Risks

No response

[jnyrup]

An alternative API

public class ObjectAssertions : ObjectAssertions<object, ObjectAssertions>
{
    public AndConstraint<ObjectAssertions> BeOneOf<TExpectation>(IEnumerable<TExpectation> validValues, IEqualityComparer<TExpectation> comparer, string because = "", params object[] becauseArgs);
}

public class ObjectAssertions<TSubject, TAssertions> : ReferenceTypeAssertions<TSubject, TAssertions>
    where TAssertions : ObjectAssertions<TSubject, TAssertions>
{
    public AndConstraint<TAssertions> BeOneOf(IEnumerable<TSubject> validValues, IEqualityComparer<TSubject> comparer, string because = "", params object[] becauseArgs);
}

The difference comes for classes deriving from ObjectAssertions<TSubject, TAssertions> e.g. HttpResponseMessageAssertions. The proposed API would allow HttpResponseMessageAssertions.BeOneOf() to take input of arbitrary types, while it currently only allows input of HttpResponseMessage.

[jnyrup]

If TSubject imlements IEquatable<TSubject> I would hope that providing IEqualityComparer<TSubject> is optional?

If not providing an IEqualityComparer, wouldn’t the existing BeOneOf suffice? See this example: https://dotnetfiddle.net/N2Rpt6