How Can I Compare Two Lists for Equality, Ignoring Element Order?

Ignore element order for list equivalence comparisons

When comparing two List<T> objects, you usually want to check whether their elements are equal, regardless of their position in the list. Here are a few ways to accomplish this.

1. Element equality and number of occurrences

If you need two lists to contain the same elements with the same number of occurrences, it is recommended to sort before comparing:

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

2. Optimization method

To improve performance, especially when dealing with large lists, you can use this custom method, which only needs to implement IEquatable:

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.ContainsKey(s) ? cnt[s] + 1 : 1;
    }
    foreach (T s in list2)
    {
        if (cnt.ContainsKey(s))
        {
            cnt[s]--;
        }
        else
        {
            return false;
        }
    }
    return cnt.Values.All(c => c == 0);
}

3. Handling nullable types

If your element is of a nullable type, you can use a dictionary's custom comparator:

var comparer = EqualityComparer<T>.Default;
var cnt = new Dictionary<T, int>(comparer);

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