gistfile1.txt

using System;
using System.Collections.Generic;

namespace Collections2
{
    internal class Program
    {
        static void Main(string[] args)
        {
            bool isOpen = true;

            string commandExit = "exit";
            string commandSum = "sum";
            string userInput;

            int userInputNumber;

            List<int> numbers = new List<int>();

            while (isOpen)
            {
                Console.Clear();

                Console.Write("Ввеите число или команду: ");
                userInput = Console.ReadLine();

                if (userInput == commandSum)
                {
                    WriteSum(numbers);
                }
                else if (userInput == commandExit)
                {
                    isOpen = false;
                }
                else if (int.TryParse(userInput, out userInputNumber))
                {
                    numbers.Add(userInputNumber);
                }

                Console.ReadKey();
            }
        }

        private static void WriteSum(List<int> numbers)
        {
            int sum = 0;

            foreach (int number in numbers)
            {
                sum += number;
            }

            Console.WriteLine($"Сумма всех чисел равна: {sum}.");
            Console.ReadKey();
        }
    }
}