The user enters the numbers, and the program remembers them. When user enters the sum command, the program outputs the sum of all the entered numbers. The program exits only if the user enters the exit command.

Dynamic array

namespace MyCode
{
    internal class Program
    {
        static void Main(string[] args)
        {
            const string CommandSum = "sum";
            const string CommandExit = "exit";

            int[] ints = new int[0];
            int sumTextLine = 15;
            int enterTextLine = 10;
            string enter;
            bool isRunning = true;

            while (isRunning)
            {
                Console.WriteLine($"Enter {CommandSum} to get sum of entered numbers.\n" +
                    $"Enter {CommandExit} to exit the program.");
                Console.WriteLine("\n\nAlready entered numbers: ");

                for (int i = 0; i < ints.Length; i++)
                {
                    Console.Write(ints[i] + " ");
                }

                Console.SetCursorPosition(0, enterTextLine);

                switch (enter = Console.ReadLine())
                {
                    case CommandSum:
                        int sum = 0;

                        if (ints != null || ints.Length != 0)
                        {
                            for (int i = 0; i < ints.Length; i++)
                            {
                                sum += ints[i];
                            }

                            Console.SetCursorPosition(0, sumTextLine);
                            Console.WriteLine("Your sum is " + sum);
                            break;
                        }

                        Console.WriteLine(sum);
                        break;

                    case CommandExit:
                        isRunning = false;
                        break;

                    default:
                        bool correct = false;

                        foreach (char c in enter)
                        {
                            if (char.IsDigit(c))
                            {
                                correct = true;
                            }
                            else
                            {
                                correct = false;
                            }
                        }

                        if (correct)
                        {
                            int[] tempNumberList = new int[ints.Length + 1];

                            for (int i = 0; i < ints.Length; i++)
                            {
                                tempNumberList[i] = ints[i];
                            }

                            tempNumberList[tempNumberList.Length - 1] = int.Parse(enter);
                            ints = tempNumberList;
                            break;
                        }

                        Console.WriteLine("\nYour enter is incorrect! Enter command or a digit!");
                        break;
                }

                Console.WriteLine("\n\nPress any button to continue...");
                Console.ReadKey();
                Console.Clear();
            }
        }
    }
}