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

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.