https://docs.google.com/document/d/1MPmU4xRsuCRyn1FVTSbeeeOpfXJ1x0XVXvl9SUZilXc の3問目

ゼロが入力されるまで入力された数を足し続ける.cs

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        Console.Clear();
        Console.WriteLine($"Enter digit numbers ({int.MinValue}~{int.MaxValue}).");
        Console.WriteLine($"I'll totaling those numbers until enter 0.");

        var sum = EnumEnteredNumbers()
            .TakeWhile(n => n != 0)
            .Sum();

        Console.WriteLine($"Totaled number is: {sum}");
    }

    static IEnumerable<int> EnumEnteredNumbers()
    {
        for (;;)
        {
            Console.Write("> ");
            var line = Console.ReadLine();
            var num = default(int);
            if (int.TryParse(line, out num))
                yield return num;
            else
                Console.WriteLine($"ERROR: Enter digit number ({int.MinValue}~{int.MaxValue})");
        }
    }
}