using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<int> list1 = new List<int>{1, 2, 3, 4, 5, 6};

        List<int> list2 = new List<int>{1, 2, 3};

        List<int> list3 = new List<int>{1, 2};

        var lists = new IEnumerable<int>[]{list1, list2, list3};

        var commons = ExistsInAll(lists);

        Console.WriteLine("Common integers:");

        foreach (var c in commons)

            Console.WriteLine(c);

    }

​

    static IEnumerable<int> ExistsInAll(IEnumerable<int>[] lists)

    {

        HashSet<int> hs = new HashSet<int>(lists.First());

        for (int i = 1; i < lists.Length; i++)

            hs.IntersectWith(lists[i]);

        return hs;

    }

}