How to create a HashSet with distinct elements?

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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();
}

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