using System;

​

public static class SumOfArray

{

    public static int CalculateSum(int[] array)

    {

        return CalculateSum(array, 0);

    }

    private static int CalculateSum(int[] array, int index)

    {

        if (index >= 0 && index < array.Length)

        {

            // when inside the array

            return array[index] + CalculateSum(array, index + 1);

        }

        // when outside of the array

        return 0;

    }

}

​

public class Program

{

    public static void Main()

    {

        int[] vector = {1, 2, 3, 4};

        int result = SumOfArray.CalculateSum(vector);

        Console.WriteLine("The result is: " + result);

    }

}