c# - How to create a HashSet<List<Int>> with distinct elements?

Question

I have a HashSet that contains multiple lists of integers - i.e. HashSet<List<int>>

In order to maintain uniqueness I am currently having to do two things: 1. Manually loop though existing lists, looking for duplicates using SequenceEquals. 2. Sorting the individual lists so that SequenceEquals works currently.

Is there a better way to do this? Is there an existing IEqualityComparer that I can provide to the HashSet so that HashSet.Add() can automatically handle uniqueness?

var hashSet = new HashSet<List<int>>();

for(/* some condition */)
{
    List<int> list = new List<int>();

    ...

    /* for eliminating duplicate lists */

    list.Sort();

    foreach(var set in hashSet)
    {
        if (list.SequenceEqual(set))
        {
            validPartition = false;
            break;
        }
    }

    if (validPartition)
           newHashSet.Add(list);
}

Answer 1

This starts off wrong, it has to be a HashSet<ReadOnlyCollection<>> because you cannot allow the lists to change and invalidate the set predicate. This then allows you to calculate a hash code in O(n) when you add the collection to the set. And an O(n) test to check if it is already in the set with a very uncommon O(n^2) worst case if all the hashes turn out to be equal. Store the computed hash with the collection.