Project euler 8: weird issue

Question

I've tried solving Project Euler Problem 8 I believe I have the right approach, but for some reason I get the exception ("An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll") in below code when i=494. Line is 38 I believe.
PS. The length of the string is 1000 as shown by debugger too.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace euler8
{
    class Program
    {
        static void Main(string[] args)
        {
           const String candidate =   "73167176531330624919225119674426574742355349194934"+
                                      "96983520312774506326239578318016984801869478851843"+
                                      "85861560789112949495459501737958331952853208805511"+
                                      "12540698747158523863050715693290963295227443043557"+
                                      "66896648950445244523161731856403098711121722383113"+
                                      "62229893423380308135336276614282806444486645238749"+
                                      "30358907296290491560440772390713810515859307960866"+
                                      "70172427121883998797908792274921901699720888093776"+
                                      "65727333001053367881220235421809751254540594752243"+
                                      "52584907711670556013604839586446706324415722155397"+
                                      "53697817977846174064955149290862569321978468622482"+
                                      "83972241375657056057490261407972968652414535100474"+
                                      "82166370484403199890008895243450658541227588666881"+
                                      "16427171479924442928230863465674813919123162824586"+
                                      "17866458359124566529476545682848912883142607690042"+
                                      "24219022671055626321111109370544217506941658960408"+
                                      "07198403850962455444362981230987879927244284909188"+
                                      "84580156166097919133875499200524063689912560717606"+
                                      "05886116467109405077541002256983155200055935729725"+
                                      "71636269561882670428252483600823257530420752963450";

            long max = 0;
            int length = candidate.Length;
            for (int i = 0; i < length - 13; i++)
            {
                string substring = candidate.Substring(i, i + 13);

                int [] x = new int [13];
                for(int j = 0; j<13; j++)
                    x[j]= int.Parse(substring[j].ToString());

                long product = 1;
                for (int k = 0; k < 13; k++)
                    product = product * x[k];

                if (product > max)
                    max = product;

            }

            Console.WriteLine(max.ToString());



        }
    }
}

Does anyone spot the issue?

Answer 1

String.Substring needs a starting position plus a length, not and end position.

So just use

string substring = candidate.Substring(i, 13);

Answer 2

It looks like this is stemming from a misunderstanding of how .SubString() works.

string substring = candidate.Substring(i, i + 13);

Most likely should be:

string substring = candidate.Substring(i, 13);

.SubString() is : .SubString(startPosition, length) So adding i + 13 doesn't net you the next 13 characters, but instead the next i + 13 characters from start position i.