set equality in linq

Question

I have two lists A and B (List). How to determine if they are equal in the cheapest way? I can write something like '(A minus B) union (B minus A) = empty set' or join them together and count amount of elements, but it is rather expensive. Is there workaround?

Answer 1

If the ordering of the list items is relevant:

bool areEqual = a.SequenceEqual(b);

If the lists are to be treated as unordered sets:

// assumes that the list items are ints
bool areEqual = new HashSet<int>(a).SetEquals(b);

(The SequenceEqual method and the HashSet<T> constructor both have overloads that take an IEqualityComparer<T> parameter, if you need that functionality.)

Answer 2

Well, that depends on how you interpret your lists.

If you consider them as tuples (so the order of elements in lists matters), then you can go with this code:

    public bool AreEqual<T>(IList<T> A, IList<T> B)
    {
        if (A.Count != B.Count)
            return false;
        for (int i = 0; i < A.Count; i++)
            if (!A[i].Equals(B[i])) 
                return false;
    }

If you consider your lists as sets (so the order of elements doesn't matter), then... you are using the wrong data structures I guess:

    public bool AreEqual<T>(IList<T> A, IList<T> B)
    {
        HashSet<T> setA = new HashSet<T>(A);
        return setA.SetEquals(B);
    }