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?

Answer 1

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>() .

Answer 2

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;
}