Can I test two ArrayList (C#) instances for value equality?

Question

I wrote a small helper method contentsToArrayList. If I pass it an object which implements ICollection, it returns an ArrayList containing the same elements as the original object. If I pass it another object, it returns an ArrayList containing the object itself.

Now I want to test the method. My unit test looks like this (a bit abbreviated, I included more test cases):

        //Arrange
        int a = 1;
        ArrayList aAsArrayList = new ArrayList();
        aAsArrayList.Add(a);
        List<int> f = new List<int>() { 4, 5, 6 };
        ArrayList fAsArrayList = new ArrayList(f);

        //Act
        ArrayList aReturned = contentsToArrayList(a);
        ArrayList fReturned = contentsToArrayList(f);

Now I am not sure how to write my asserts. Basically, I want to make sure that aAsArrayList contains the same object(s) as aReturned. But ArrayList being a reference type, I am not sure it has value equality defined. Can I just compare the arraylists easily, using something like aReturned == aAsArrayList, or do I have to compare each member of the arraylist?

Are you using nunit? If so use CollectionAssert http://www.nunit.org/index.php?p=collectionAssert&r=2.4 — doctorlove, Aug 05 '13 at 10:22
@doctorlove No, I am using the built-in test framework in Visual Studio. But thank you for the nunit solution. — Rumi P., Aug 05 '13 at 10:42

score 4 · Accepted Answer · answered Aug 05 '13 at 10:14

4

I am not sure it has value equality defined.

You are correct, it does not have value equality defined.

One way to compare ArrayList objects for equality is using LINQ:

if (aReturned.Cast<object>().SequenceEqual(fReturned.Cast<object>())) {
    ...
}

Casts to object are the most "forgiving" - they would not trigger a cast exception in your tests. If you know that both arrays contain values of type int, you can replace the type in the Cast<object>() to Cast<int>().

answered Aug 05 '13 at 10:14

Sergey Kalinichenko

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Thank you. The SequenceEqual method is the kind of solution I had in mind but did not know if it exists. – Rumi P. Aug 05 '13 at 10:41

score 1 · Answer 2 · edited May 23 '17 at 12:30

1

A similar question about the C# specification

List<T> operator == In the C# Language Specification Version 4 on StackOverflow

Would imply that the answer to this question is no

Additionally the page

ArrayList on MSDN

Shows that the ArrayList object has no override for == operator

Thus you will probably write a helper method that checks that each element of one array is contained within the other. If this is true for both ArrayLists then they are equal in the way you desire. Something like

public bool ArrayListsEqual(ArrayList a, ArrayList b) {
    foreach (obj A in a) {
        if (!b.Contains(A)) {
            return false;
        }
    }
    foreach (obj B in b) {
        if (!a.Contains(B)) {
            return false;
        }
    }
    return true;
}

edited May 23 '17 at 12:30

Community

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answered Aug 05 '13 at 10:12

James Robinson

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FYI `==` is not a method it is an `operator` and your `ArrayListsEqual` is inefficient (O(n1) * O(n2)) * 2 – Sriram Sakthivel Aug 05 '13 at 10:27
1

Of course it is inefficent. When writing a test helper method the priority is that the logic is sound and clearly so so one doesn't end up needing to write tests for ones tests. – James Robinson Aug 05 '13 at 10:28
Yes but why to break up our head when framework provides one which is efficient `SequenceEqual`. – Sriram Sakthivel Aug 05 '13 at 10:31
Additionally I believe Sequence equal checks the order of the items is identical, which (from the question) does not appear to be required behaviour. – James Robinson Aug 05 '13 at 10:55
Yup, you're right. In that case you can sort both lists and use `SequenceEqual` isn't it? – Sriram Sakthivel Aug 05 '13 at 10:57

Can I test two ArrayList (C#) instances for value equality?

2 Answers2