MSTest: CollectionAssert.AreEquivalent failed. The

Question

Question:

Can anyone tell me why my unit test is failing with this error message?

CollectionAssert.AreEquivalent failed. The expected collection contains 1 occurrence(s) of . The actual collection contains 0 occurrence(s).

Goal:

I'd like to check if two lists are identical. They are identical if both contain the same elements with the same property values. The order is irrelevant.

Code example:

This is the code which produces the error. list1 and list2 are identical, i.e. a copy-paste of each other.

[TestMethod]
public void TestListOfT()
{
    var list1 = new List<MyPerson>()
    {
        new MyPerson()
        {
            Name = "A",
            Age = 20
        },
        new MyPerson()
        {
            Name = "B",
            Age = 30
        }
    };
    var list2 = new List<MyPerson>()
    {
        new MyPerson()
        {
            Name = "A",
            Age = 20
        },
        new MyPerson()
        {
            Name = "B",
            Age = 30
        }
    };

    CollectionAssert.AreEquivalent(list1.ToList(), list2.ToList());
}

public class MyPerson
{
    public string Name { get; set; }
    public int Age { get; set; }
}

I've also tried this line (source)

CollectionAssert.AreEquivalent(list1.ToList(), list2.ToList());

and this line (source)

CollectionAssert.AreEquivalent(list1.ToArray(), list2.ToArray());

P.S.

Related Stack Overflow questions:

I've seen both these questions, but the answers didn't help.

• CollectionAssert use with generics?
• Unit-testing IList with CollectionAssert

Answer 1

It works if I add an IEqualityComparer<T> as described on MSDN and if I use Enumerable.SequenceEqual. Note however, that now the order of the elements is relevant.

In the unit test

//CollectionAssert.AreEquivalent(list1, list2); // Does not work
Assert.IsTrue(list1.SequenceEqual(list2, new MyPersonEqualityComparer())); // Works

IEqualityComparer

public class MyPersonEqualityComparer : IEqualityComparer<MyPerson>
{
    public bool Equals(MyPerson x, MyPerson y)
    {
        if (object.ReferenceEquals(x, y)) return true;

        if (object.ReferenceEquals(x, null) || object.ReferenceEquals(y, null)) return false;

        return x.Name == y.Name && x.Age == y.Age;
    }

    public int GetHashCode(MyPerson obj)
    {
        if (object.ReferenceEquals(obj, null)) return 0;

        int hashCodeName = obj.Name == null ? 0 : obj.Name.GetHashCode();
        int hasCodeAge = obj.Age.GetHashCode();

        return hashCodeName ^ hasCodeAge;
    }
}

Answer 2

You are absolutely right. Unless you provide something like an IEqualityComparer<MyPerson> or implement MyPerson.Equals(), the two MyPerson objects will be compared with object.Equals, just like any other object. Since the objects are different, the Assert will fail.

Answer 3

I wrote this to test collections where the order is not important:

    public static bool AreCollectionsEquivalent<T>(ICollection<T> collectionA, ICollection<T> collectionB, IEqualityComparer<T> comparer)
    {
        if (collectionA.Count != collectionB.Count)
            return false;

        foreach (var a in collectionA)
        {
            if (!collectionB.Any(b => comparer.Equals(a, b)))
                return false;
        }

        return true;
    }

Not as elegant as using SequenceEquals, but it works.

Of course to use it you simply do:

Assert.IsTrue(AreCollectionsEquivalent<MyType>(collectionA, collectionB, comparer));

Answer 4

I was getting this same error when testing a collection persisted by nHibernate. I was able to get this to work by overriding both the Equals and GetHashCode methods. If I didn't override both I still got the same error you mentioned:

CollectionAssert.AreEquivalent failed. The expected collection contains 1 occurrence(s) of . 
The actual collection contains 0 occurrence(s).

I had the following object:

public class EVProjectLedger
{
    public virtual long Id { get; protected set; }
    public virtual string ProjId { get; set; }
    public virtual string Ledger { get; set; }
    public virtual AccountRule AccountRule { get; set; }
    public virtual int AccountLength { get; set; }
    public virtual string AccountSubstrMethod { get; set; }

    private Iesi.Collections.Generic.ISet<Contract> myContracts = new HashedSet<Contract>();

    public virtual Iesi.Collections.Generic.ISet<Contract> Contracts
    {
        get { return myContracts; }
        set { myContracts = value; }
    }

    public override bool Equals(object obj)
    {
        EVProjectLedger evProjectLedger = (EVProjectLedger)obj;
        return ProjId == evProjectLedger.ProjId && Ledger == evProjectLedger.Ledger;
    }
    public override int GetHashCode()
    {
        return new { ProjId, Ledger }.GetHashCode();
    }
}

Which I tested using the following:

using (ITransaction tx = session.BeginTransaction())
{
    var evProject = session.Get<EVProject>("C0G");

    CollectionAssert.AreEquivalent(TestData._evProjectLedgers.ToList(), evProject.EVProjectLedgers.ToList());

    tx.Commit();
}

I'm using nHibernate which encourages overriding these methods anyways. The one drawback I can see is that my Equals method is based on the business key of the object and therefore tests equality using the business key and no other fields. You could override Equals however you want but beware of equality pollution mentioned in this post:

CollectionAssert.AreEquivalent failing... can't figure out why

Answer 5

If you would like to achieve this without having to write an equality comaparer, there is a unit testing library that you can use, called FluentAssertions,

https://fluentassertions.com/documentation/

It has many built in equality extension functions including ones for the Collections. You can install it through Nuget and its really easy to use.

Taking the example in the question above all you have to write in the end is

list1.Should().BeEquivalentTo(list2);

By default, the order matters in the two collections, however it can be changed as well.