Lesson: List

using System;
using System.Collections.Generic;

namespace Collections
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> numbers = new List<int>();
            int summ = 0;
            string input;
            bool exit = false;

            while (exit != true)
            {
                input = Console.ReadLine();

                switch (input)
                {
                    case "sum":
                        SumNumbers(summ, numbers);
                        break;
                    case "exit":
                        exit = true;
                        break;
                    default:
                        NumbersChek(input, numbers);
                        break;
                }
            }
        }

        static void SumNumbers(int summ, List<int> numbers)
        {
            for (int i = 0; i < numbers.Count; i++)
            {
                summ += numbers[i];
            }
            Console.WriteLine("Сумма введенных чисел равна: " + summ);
            summ = 0;
        }

        static void NumbersChek(string input, List<int> numbers)
        {

            if (int.TryParse(input, out int number))
            {
                numbers.Add(number);
            }
            else
            {
                Console.WriteLine("Вы ввели неверную команду!");
            }
        }
    }
}