using System;

using System.Collections.Generic;

using System.Linq;

public class MyComparer : IEqualityComparer<List<int>> {

  public bool Equals(List<int> A, List<int> B) {

    if (ReferenceEquals(A, B))

      return true;

    if (A is null || B is null)

      return false;

​

    return A.SequenceEqual(B);

  }

​

  public int GetHashCode(List<int> list) => list is null

    ? -1

    : list.Count;

}

​

public class Program {

  public static void Main() {

    var hs = new HashSet<List<int>>(new MyComparer());

    List<int> list = new List<int>() { 1, 2, 3 };

    hs.Add(new List<int>(){1, 2, 3});

    hs.Add(list);

    Console.WriteLine($"{hs.Count}");   

  }

}