How Can I Efficiently Compare Two Unordered Lists for Equality in C#?

Compare unordered List objects for equality

Determining whether two List objects are equal is a common problem. Although they may contain the same elements, their order may be different. This begs the question: how to compare elements regardless of their order.

If the number of occurrences of each element is important, a simple solution is to sort both lists before comparing:

Enumerable.SequenceEqual(list1.OrderBy(t => t), list2.OrderBy(t => t))

Optimization solution for IEquatable elements

For elements that only need to implement the IEquatable interface (instead of IComparable), there is a more efficient approach:

public static bool ScrambledEquals<T>(IEnumerable<T> list1, IEnumerable<T> list2) {
    var cnt = new Dictionary<T, int>();
    foreach (T s in list1) {
        cnt[s] = cnt.GetValueOrDefault(s, 0) + 1;
    }
    foreach (T s in list2) {
        if (cnt.ContainsKey(s)) cnt[s] -= 1;
        else return false;
    }
    return cnt.Values.All(x => x == 0);
}

This scheme counts the number of occurrences of each element in the first list and then subtracts the number of occurrences from the second list. The two lists are considered equal if each count reaches zero.

Handling custom key types

If the element types are not comparable (e.g., nullable types), you can specify a comparator for the dictionary:

public static bool ScrambledEquals<T>(IEnumerable<T> list1, IEnumerable<T> list2, IEqualityComparer<T> comparer) {
    var cnt = new Dictionary<T, int>(comparer);
    ...
}

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