How does Assert.AreEqual determine equality between two generic IEnumerables?

Question

I have a unit test to check whether a method returns the correct IEnumerable. The method builds the enumerable using yield return. The class that it is an enumerable of is below:

enum TokenType
{
    NUMBER,
    COMMAND,
    ARITHMETIC,
}

internal class Token
{
    public TokenType type { get; set; }
    public string text { get; set; }
    public static bool operator == (Token lh, Token rh) { return (lh.type == rh.type) && (lh.text == rh.text); }
    public static bool operator != (Token lh, Token rh) { return !(lh == rh); }
    public override int GetHashCode()
    {
        return text.GetHashCode() % type.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return this == (Token)obj;
    }
}

This is the relevant part of the method:

 foreach (var lookup in REGEX_MAPPING)
 {
     if (lookup.re.IsMatch(s))
     {
         yield return new Token { type = lookup.type, text = s };
         break;
     }
 }

If I store the result of this method in actual, make another enumerable expected, and compare them like this...

  Assert.AreEqual(expected, actual);

..., the assertion fails.

I wrote an extension method for IEnumerable that is similar to Python's zip function (it combines two IEnumerables into a set of pairs) and tried this:

foreach(Token[] t in expected.zip(actual))
{
    Assert.AreEqual(t[0], t[1]);
}

It worked! So what is the difference between these two Assert.AreEquals?

Answer 1

Found it:

Assert.IsTrue(expected.SequenceEqual(actual));

Answer 2

Have you considered using the CollectionAssert class instead...considering that it is intended to perform equality checks on collections?

Addendum:
If the 'collections' being compared are enumerations, then simply wrapping them with 'new List<T>(enumeration)' is the easiest way to perform the comparison. Constructing a new list causes some overhead of course, but in the context of a unit test this should not matter too much I hope?

Answer 3

Assert.AreEqual is going to compare the two objects at hand. IEnumerables are types in and of themselves, and provide a mechanism to iterate over some collection...but they are not actually that collection. Your original comparison compared two IEnumerables, which is a valid comparison...but not what you needed. You needed to compare what the two IEnumerables were intended to enumerate.

Here is how I compare two enumerables:

Assert.AreEqual(t1.Count(), t2.Count());

IEnumerator<Token> e1 = t1.GetEnumerator();
IEnumerator<Token> e2 = t2.GetEnumerator();

while (e1.MoveNext() && e2.MoveNext())
{
    Assert.AreEqual(e1.Current, e2.Current);
}

I am not sure whether the above is less code than your .Zip method, but it is about as simple as it gets.

Answer 4

I think the simplest and clearest way to assert the equality you want is a combination of the answer by jerryjvl and comment on his post by MEMark - combine CollectionAssert.AreEqual with extension methods:

CollectionAssert.AreEqual(expected.ToArray(), actual.ToArray());

This gives richer error information than the SequenceEqual answer suggested by the OP (it will tell you which element was found that was unexpected). For example:

IEnumerable<string> expected = new List<string> { "a", "b" };
IEnumerable<string> actual   = new List<string> { "a", "c" }; // mismatching second element

CollectionAssert.AreEqual(expected.ToArray(), actual.ToArray());
// Helpful failure message!
//  CollectionAssert.AreEqual failed. (Element at index 1 do not match.)    

Assert.IsTrue(expected.SequenceEqual(actual));
// Mediocre failure message:
//  Assert.IsTrue failed.   

You'll be really pleased you did it this way if/when your test fails - sometimes you can even know what's wrong without having to break out the debugger - and hey you're doing TDD right, so you write a failing test first, right? ;-)

The error messages get even more helpful if you're using AreEquivalent to test for equivalence (order doesn't matter):

CollectionAssert.AreEquivalent(expected.ToList(), actual.ToList());
// really helpful error message!
//  CollectionAssert.AreEquivalent failed. The expected collection contains 1
//  occurrence(s) of <b>. The actual collection contains 0 occurrence(s).