MSTest: CollectionAssert.AreEquivalent failed. The expected collection contains 1 occurrence(s) of

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

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 2

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;
    }
}