using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        HashSet<int> numbers1 = new HashSet<int>();     

        numbers1.Add(1);

        numbers1.Add(2);

        numbers1.Add(3);

        HashSet<int> numbers2 = new HashSet<int>();     

        numbers2.Add(4);

        numbers2.Add(5);

        var sets = new HashSet<HashSet<int>>();

        sets.Add(numbers1);

        sets.Add(numbers2);

        var product = CartesianProduct(sets);

        // I expected product to contain: [[1,4], [1,5], [2,4], [2,5] ...]

    }

    private static HashSet<HashSet<int>> CartesianProduct(HashSet<HashSet<int>> sets) {

        //...

    }

}