This challenge wants you to find the largest product in a 20x20 grid that is 4 numbers in all directions (up, down, left, right, and all diagonals). This was pretty straight forward, I went with a brute force method.

namespace ProjectEuler_11
{
    internal class Program
    {
        private static int[,] _data =
        {
            {08,02,22,97,38,15,00,40,00,75,04,05,07,78,52,12,50,77,91,08},
            {49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,04,56,62,00},
            {81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,03,49,13,36,65},
            {52,70,95,23,04,60,11,42,69,24,68,56,01,32,56,71,37,02,36,91},
            {22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80},
            {24,47,32,60,99,03,45,02,44,75,33,53,78,36,84,20,35,17,12,50},
            {32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70},
            {67,26,20,68,02,62,12,20,95,63,94,39,63,08,40,91,66,49,94,21},
            {24,55,58,05,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72},
            {21,36,23,09,75,00,76,44,20,45,35,14,00,61,33,97,34,31,33,95},
            {78,17,53,28,22,75,31,67,15,94,03,80,04,62,16,14,09,53,56,92},
            {16,39,05,42,96,35,31,47,55,58,88,24,00,17,54,24,36,29,85,57},
            {86,56,00,48,35,71,89,07,05,44,44,37,44,60,21,58,51,54,17,58},
            {19,80,81,68,05,94,47,69,28,73,92,13,86,52,17,77,04,89,55,40},
            {04,52,08,83,97,35,99,16,07,97,57,32,16,26,26,79,33,27,98,66},
            {88,36,68,87,57,62,20,72,03,46,33,67,46,55,12,32,63,93,53,69},
            {04,42,16,73,38,25,39,11,24,94,72,18,08,46,29,32,40,62,76,36},
            {20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,04,36,16},
            {20,73,35,29,78,31,90,01,74,31,49,71,48,86,81,16,23,57,05,54},
            {01,70,54,71,83,51,54,69,16,92,33,48,61,43,52,01,89,19,67,48}
        };

        private static int GetLargerHorizontalProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct)
        {
            // make sure we have enough colums to get the number of values we need
            if (x + multipleCount >= cols)
            {
                return largestProduct;
            }

            int product = data[x, y];

            for (int i = x + 1; i < multipleCount + x; i++)
            {
                product *= data[i, y];
            }

            return product > largestProduct ? product : largestProduct;
        }

        private static int GetLargerLeftDiagnolProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct)
        {
            if (x + multipleCount >= cols || y - multipleCount < 0)
            {
                return largestProduct;
            }

            int product = data[x, y];

            for (int i = x + 1, j = y - 1; i < x + multipleCount; i++, j--)
            {
                product *= data[i, j];
            }

            return product > largestProduct ? product : largestProduct;
        }

        private static int GetLargerProduct(int multipleCount, int[,] data, int rows, int cols, int largestProduct, int x, int y)
        {
            largestProduct = GetLargerHorizontalProduct(multipleCount, data, x, y, rows, cols, largestProduct);
            largestProduct = GetLargerVerticalProduct(multipleCount, data, x, y, rows, cols, largestProduct);
            largestProduct = GetLargerRightDiagnolProduct(multipleCount, data, x, y, rows, cols, largestProduct);
            largestProduct = GetLargerLeftDiagnolProduct(multipleCount, data, x, y, rows, cols, largestProduct);
            return largestProduct;
        }

        private static int GetLargerRightDiagnolProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct)
        {
            // make sure we have enough room to get the number of values we need
            if (x + multipleCount >= cols || y + multipleCount >= rows)
            {
                return largestProduct;
            }

            int product = data[x, y];

            for (int i = x + 1, j = y + 1; i < x + multipleCount; i++, j++)
            {
                product *= data[i, j];
            }

            return product > largestProduct ? product : largestProduct;
        }

        private static int GetLargerVerticalProduct(int multipleCount, int[,] data, int x, int y, int rows, int cols, int largestProduct)
        {
            // make sure we have enough rows to get the number of values we need
            if (y + multipleCount >= rows)
            {
                return largestProduct;
            }

            int product = data[x, y];

            for (int i = y + 1; i < y + multipleCount; i++)
            {
                product *= data[x, i];
            }

            return product > largestProduct ? product : largestProduct;
        }

        private static void Main(string[] args)
        {
            const int multipleCount = 4;
            int rows = _data.GetLength(0);
            int cols = _data.GetLength(1);
            int largestProduct = 0;

            for (int x = 0; x < rows; x++)
            {
                for (int y = 0; y < cols; y++)
                {
                    largestProduct = GetLargerProduct(multipleCount, _data, rows, cols, largestProduct, x, y);
                }
            }

            System.Console.WriteLine("Largest multiple found: {0}", largestProduct);
        }
    }
}