Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

FlattenList.cs

using System;
using System.Collections;
using System.Collections.Generic;

namespace Flatten
{
    class Program
    {
        static void Main(string[] args)
        {
            //I try my best to avoid static methods unless I really have to so I did everything under an instance. :)
            var instance = new Program();
        }

        public Program()
        {
            Init();
        }

        void Init()
        {
            var numbersToParse = new ArrayList{
                1,2, new ArrayList {
                    3,4,
                    new ArrayList
                    {
                        5,6,
                        new ArrayList
                        {
                            20, 40
                        }
                    }
                }
            };

            var flattenedList = FlattenToString(numbersToParse);

            foreach(var nextInt in flattenedList)
                Console.WriteLine($"{nextInt}");

            Console.WriteLine("The End! :)");
        }

        /// <summary>
        /// Flattens an ArrayList of ArrayList or int into a one level List
        /// </summary>
        /// <returns>Flattened List</returns>
        /// <param name="toFlatten">ArrayList to flatten.</param>
        IList<int> FlattenToString(ArrayList toFlatten)
        {
            var flattenedList = new List<int>();
                
            foreach(var nextValue in toFlatten)
            {
                if(nextValue is ArrayList nextValueToArrayList)
                {
                    var result = FlattenToString(nextValueToArrayList);

                    flattenedList.AddRange(result);
                }
                else if (nextValue is int nextValueToInt)
                {
                    flattenedList.Add(nextValueToInt);
                }
                else
                {
                    throw new Exception("Ups, I was expecting ArrayList with int or another ArrayList nested.");
                }
            }

            return flattenedList;
        }
    }
}