using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int[] numbers = new int[] { 2, 3 };

        Console.WriteLine(String.Join(",",HowSum(7, numbers)));

    }

    static int[] HowSum(int targetSum, int[] numbers)

    {

        if (targetSum == 0) return numbers;

        if (targetSum < 0) return null;

​

        foreach( var num in numbers){

            var remainder = targetSum - num;

            int[] remainderResult = HowSum(remainder, numbers);

​

            if (remainderResult != null) { 

                return remainderResult.Append(num).ToArray();

            }

        }

        return null;

    }

}

​