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

Here is a possible comparer that compares an IEnumerable<T> by its elements. You still need to sort manually before adding.

One could build the sorting into the comparer, but I don’t think that’s a wise choice. Adding a canonical form of the list seems wiser.

This code will only work in .net 4 since it takes advantage of generic variance. If you need earlier versions you need to either replace IEnumerable with List, or add a second generic parameter for the collection type.

class SequenceComparer<T>:IEqualityComparer<IEnumerable<T>>
{
    public bool Equals(IEnumerable<T> seq1,IEnumerable<T> seq2)
    {
        return seq1.SequenceEqual(seq2);
    }
    
    public int GetHashCode(IEnumerable<T> seq)
    {
        int hash = 1234567;
        foreach(T elem in seq)
            hash = unchecked(hash * 37 + elem.GetHashCode());
        return hash;
    }
}

void Main()
{
    var hashSet = new HashSet<List<int>>(new SequenceComparer<int>());

    List<int> test=new int[]{1,3,2}.ToList();
    test.Sort();
    hashSet.Add(test);

    List<int> test2=new int[]{3,2,1}.ToList();
    test2.Sort();       
    hashSet.Contains(test2).Dump();
}