Flatten int array

flatten.cs

using System;
using System.Collections.Generic;

namespace Algoritmo1
{
    class Program
    {
        static void Main(string[] args)
        {
            int[,] testArray = new int[,] { { 1, 2 }, { 3, 4 } } ;
            int[] testResult = FlattenArray(testArray);

            Console.WriteLine("Gist applying to the job \"Experienced Backend Engineer\" at Cytrusbyte");

            foreach(int i in testResult)
            {
                Console.WriteLine(i);
            }

            Console.WriteLine("Finish processing, thanks for watch.");
            Console.ReadLine();
        }

        /// <summary>
        /// Recursive flattening method
        /// </summary>
        /// <param name="arrayItem">Array of int or nested int array</param>
        /// <returns>Flattened array</returns>
        static int[] FlattenArray(dynamic arrayItem)
        {
            List<int> listReturn = new List<int>();

            if (arrayItem.GetType() == typeof(int))
                listReturn.Add((int)arrayItem);
            else
            {
                foreach (dynamic arrItem in arrayItem)
                {
                    if (arrItem.GetType() == typeof(int))
                        listReturn.Add(arrItem);
                    else
                        FlattenArray(arrItem);
                }
            }

            return listReturn.ToArray();
        }
    }    
}