ProjectEuler8.cs

//The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.
//Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

var digits = 13;
var numStr = "73167176531330624919225119674426574742355349194934
              96983520312774506326239578318016984801869478851843
              85861560789112949495459501737958331952853208805511
              12540698747158523863050715693290963295227443043557
              66896648950445244523161731856403098711121722383113
              62229893423380308135336276614282806444486645238749
              30358907296290491560440772390713810515859307960866
              70172427121883998797908792274921901699720888093776
              65727333001053367881220235421809751254540594752243
              52584907711670556013604839586446706324415722155397
              53697817977846174064955149290862569321978468622482
              83972241375657056057490261407972968652414535100474
              82166370484403199890008895243450658541227588666881
              16427171479924442928230863465674813919123162824586
              17866458359124566529476545682848912883142607690042
              24219022671055626321111109370544217506941658960408
              07198403850962455444362981230987879927244284909188
              84580156166097919133875499200524063689912560717606
              05886116467109405077541002256983155200055935729725
              71636269561882670428252483600823257530420752963450"
              
var max = Enumerable.Range(0, numStr.Length - digits).Select(idx => {return orgStr.Substring(idx, digits).Select(x => Int64.Parse(x.ToString())).ToArray().Aggregate((a, b) => a * b); }).ToArray().Max();
Console.WriteLine($"The largest product of {digits} consecutive digits is {max}");