FlattenArray.cs

using System;
using System.Collections.Generic;

namespace FlattenArray
{
    public class Program
    {
        /// <summary>
        /// Main console program method with tests cases
        /// Author: Agustín Ibars
        /// </summary>
        public static void Main()
        {
            try
            {
                // Sample [[1,2,[3]],4] -> [1,2,3,4]
                var array1 = new object[] { new object[] { 1, 2, new object[] { 3 } }, 4 };
                var result1 = Flatten(array1);
                Console.WriteLine("Flattened results:");
                Console.WriteLine("[{0}]", string.Join(", ", result1));

                var array2 = new object[] { 4 };
                var result2 = Flatten(array2);
                Console.WriteLine("[{0}]", string.Join(", ", result2));

                var array3 = new object[] { 4, new object[] { 1, 2, new object[] { 4, 5 }, 6 }, 7 };
                var result3 = Flatten(array3);
                Console.WriteLine("[{0}]", string.Join(", ", result3));

                var array4 = new object[] { 4, 9, new object[] { new object[] { 1, 2, 3 }, 4 }, new object[] { 13, "a" } }; //this should fail
                var result4 = Flatten(array4);
                Console.WriteLine("[{0}]", string.Join(", ", result4));

            }
            catch (Exception ex)
            {
                Console.WriteLine(string.Format("There was an error processing the array input\nReason: {0}", ex.Message));
            }
        }

        /// <summary>
        /// Flattens an array of arbitrarily nested arrays of integers into a flat array of integers
        /// </summary>
        /// <param name="A">An arbitrarly nested array of objects</param>
        /// <returns>A flattened integer array</returns>
        public static int[] Flatten(object[] A)
        {
            var list = new List<int>();
            FlattenHelper(ref list, A);

            return list.ToArray();
        }

        /// <summary>
        /// Recursive method that actually does the flattening of the array
        /// </summary>
        /// <param name="list">A list passed by reference so that the partial results are being added</param>
        /// <param name="A">The sub-array or main array of nested items</param>
        public static void FlattenHelper(ref List<int> list, object[] A)
        {

            for (int i = 0; i < A.Length; i++)
            {
                if (A[i].GetType() == typeof(object[]))
                {
                    FlattenHelper(ref list, A[i] as object[]);
                }
                else if (A[i].GetType() == typeof(int))
                {
                    list.Add((int)A[i]);
                }
                else
                {
                    throw new ArgumentException("The element is neither an integer nor an array of integers");
                }
            }
        }
    }
}