using System;

using System.Collections.Generic;

using System.Linq;

class Program

{

    // https://stackoverflow.com/questions/5231845/c-sharp-linq-group-by-on-multiple-columns

    class ListFirstTwoComparer : IEqualityComparer<List<int>>

    {

        public bool Equals(List<int> a, List<int> b)

        {

            var a2 = a.Take(2);

            var b2 = b.Take(2);

​

            //https://stackoverflow.com/questions/11092930/check-if-listt-contains-any-of-another-list

            return  a2.Any(x => b2.Any(y => y == x));

        }

​

        public int GetHashCode(List<int> input)

        {   // https://stackoverflow.com/questions/8094867/good-gethashcode-override-for-list-of-foo-objects-respecting-the-order

            var a2 = input.Take(2);

            unchecked

            {

                int hash = 19;

                foreach (var foo in a2)

                {

                    hash = hash * 31 + foo.GetHashCode();

                }

                return hash;

            }

        }

    }

    static void Main(string[] args)

    {

​

        var lists=new List<List<int>>(){

            new List<int>{1,2},

            new List<int>{3,2},

            new List<int>{20,21},

            new List<int>{21,22}

        };

        var result = lists.GroupBy(p => p, new ListFirstTwoComparer());

        result.Dump();

​

    }

}