gistfile1.txt

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace сум
{
    class Program
    {
        static void Main(string[] args)
        {
            int sum = 0;
            string count = "sum";
            string exit = "exit";
            string userInput = "";
            int[] Data = new int[0];

            Console.WriteLine("Введите элемент массива");

            while (true)
            {
                userInput = Console.ReadLine();
                switch (userInput)
                {
                    case "sum":
                        Console.WriteLine(String.Format("Сумма элементов = {0}", Data.Sum()));
                        break;
                    case "exit":
                        return;
                    default:
                        int result;
                        if (int.TryParse(userInput, out result))
                            Data = Data.Concat(new int[] { result }).ToArray();
                        break;
                }
            }

        }
    }
}