What is difference between EqualTo() and EquivalentTo() in NUnit?

Question

When I have a Dictionary<string, int> actual and then create a completely new Dictionary<string, int> expected with the same values as actual.

• Calling Assert.That(actual, Is.EqualTo(expected)); makes the test pass.

• When using Assert.That(actual, Is.EquivalentTo(expected)); the test doesn't pass.

What is the difference between EqualTo() and EquivalentTo()?

Edit:

The message of the exception when the test doesn't pass is as follows:

Zoozle.Tests.Unit.PredictionTests.ReturnsDriversSelectedMoreThanOnceAndTheirPositions:
Expected: equivalent to < [Michael Schumacher, System.Collections.Generic.List`1[System.Int32]] >
But was:  < [Michael Schumacher, System.Collections.Generic.List`1[System.Int32]] >

My code looks like this:

[Test]
public void ReturnsDriversSelectedMoreThanOnceAndTheirPositions()
{
    //arrange
    Prediction prediction = new Prediction();

    Dictionary<string, List<int>> expected = new Dictionary<string, List<int>>()
    {
        { "Michael Schumacher", new List<int> { 1, 2 } }
    };

    //act
    var actual = prediction.CheckForDriversSelectedMoreThanOnce();

    //assert
    //Assert.That(actual, Is.EqualTo(expected));
    Assert.That(actual, Is.EquivalentTo(expected));
}

public Dictionary<string, List<int>> CheckForDriversSelectedMoreThanOnce()
{
    Dictionary<string, List<int>> expected = new Dictionary<string, List<int>>();
    expected.Add("Michael Schumacher", new List<int> { 1, 2 });

    return expected;
}

Answer 1

The question title forces me to state the following:

For enumerations, Is.EquivalentTo() does the comparison allowing any order of the elements. In contrast, Is.EqualTo() takes into account the exact order of the elements, like Enumerable.SequenceEqual() does.

However, in your case, there is no issue with ordering. The main point here is that Is.EqualTo() has extra code for dictionary comparison, as stated here.

Not so Is.EquivalentTo(). In your example, it will compare values of type KeyValuePair<string,List<int>> for equality, using object.Equals(). Since the dictionary values are of reference type List<int>, reference equality is used for comparing them.

If you modify your example such that the List {1, 2} is only instantiated once and used in both dictionaries, Is.EquivalentTo() will succeed.