using System;

​

namespace ConsoleApp3

{

    class Program

    {

        static void Main()

        {

            string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

​

            void OutputLine(ReadOnlySpan<char> span)

            {

                for (int i = 0; i < span.Length; i++)

                {

                    Console.Write(span[i]);

                }

                Console.WriteLine();

            }

​

            IterateAllPosibleStrings(3, alphabet, OutputLine);

​

​

            Console.WriteLine("Hello World!");

        }

​

        public delegate void Action_ReadOnlySpan<T>(ReadOnlySpan<T> readOnlySpan);

​

        public static void IterateAllPosibleStrings(int length, ReadOnlySpan<char> possibleElements, Action_ReadOnlySpan<char> action)

        {

            Span<int> digits = stackalloc int[length];

            Span<char> span = stackalloc char[length];

            for (int i = 0; i < span.Length; i++)

            {

                digits[i] = 0;

                span[i] = possibleElements[0];

            }

            while (true)

            {

                action(span);

                int digit = 0;

                while (true)

                {

                    if (digit >= length)

                    {

                        return;

                    }

                    digits[digit]++;

                    if (digits[digit] >= possibleElements.Length)

                    {

                        digits[digit] = 0;

                        span[digit] = possibleElements[digits[digit]];

                        digit++;

                    }

                    else

                    {

                        span[digit] = possibleElements[digits[digit]];

                        break;

                    }

                }

            }

        }

    }

}

​