Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace _008
{
    class Program
    {
        static string BihNum = "73167176531330624919225119674426574742355349194934" +
                               "96983520312774506326239578318016984801869478851843" +
                               "85861560789112949495459501737958331952853208805511" +
                               "12540698747158523863050715693290963295227443043557" +
                               "66896648950445244523161731856403098711121722383113" +
                               "62229893423380308135336276614282806444486645238749" +
                               "30358907296290491560440772390713810515859307960866" +
                               "70172427121883998797908792274921901699720888093776" +
                               "65727333001053367881220235421809751254540594752243" +
                               "52584907711670556013604839586446706324415722155397" +
                               "53697817977846174064955149290862569321978468622482" +
                               "83972241375657056057490261407972968652414535100474" +
                               "82166370484403199890008895243450658541227588666881" +
                               "16427171479924442928230863465674813919123162824586" +
                               "17866458359124566529476545682848912883142607690042" +
                               "24219022671055626321111109370544217506941658960408" +
                               "07198403850962455444362981230987879927244284909188" +
                               "84580156166097919133875499200524063689912560717606" +
                               "05886116467109405077541002256983155200055935729725" +
                               "71636269561882670428252483600823257530420752963450";

        static int ZeroIndex(long[] arr)
        {
            for (int i = arr.Length - 1; i >= 0; i--)
                if (arr[i] == 0)
                    return i;
            return -1;
        }

        static Tuple<long[], long> GetAnswer(int num)
        {
            int[] arr = new int[1000];
            long[] arr2 = null;

            for (int i = 0; i < BihNum.Length; i++)
                arr[i] = int.Parse(BihNum[i].ToString());


            List<long> numbers = new List<long>();
            List<long[]> array = new List<long[]>();


            for (int i = 0; i < arr.Length - num - 1; i++)
            {
                arr2 = new long[num];
                for (int j = 0; j < num; j++)
                    arr2[j] = arr[i + j];

                int index = ZeroIndex(arr2);
                if (index == -1)
                {
                    numbers.Add(arr2.Aggregate((x, y) => x * y));
                    array.Add(arr2);
                }
                else if (index != 0)
                    i = i + index - 1;
            }

            long max = numbers.Max();
            int MaxIndex = numbers.IndexOf(max);

            return new Tuple<long[], long>(array[MaxIndex], max);
        }

        static void Main(string[] args)
        {
            Console.BackgroundColor = ConsoleColor.White;
            Console.ForegroundColor = ConsoleColor.Black;
            Console.Clear();
            Console.WriteLine("Project Euler. Verloka Vadim, 2019");
            Console.WriteLine(new string('-', 50));

            var answer = GetAnswer(13);

            Console.WriteLine($"Answer: {string.Join("*", answer.Item1)}={answer.Item2}");

            Console.Read();
        }
    }
}