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

namespace Excercises
{
    class Program
    {
        static void Main(string[] args)
        {
            var stopwatch = new Stopwatch();
            Console.WriteLine("Type start\\stop\\clear to use the stopwatch, x to exit");

            while (true)
            {
                var input = Console.ReadLine();

                switch (input)
                {
                    case "start":
                        stopwatch.Start();
                        break;
                    case "stop":
                        stopwatch.Stop();
                        break;
                    case "x":
                        return;
                }
            }
        }
    }

    public class Stopwatch
    {
        private DateTime _startTime;
        private DateTime _stopTime;
        private TimeSpan _duration;
        private string _StrDuration;
        private bool _startOn = false;

        public Stopwatch()
        {
            Console.WriteLine($"Stopwatch displays: {_duration}");
        }

        public DateTime Start()
        {
            if (this._startOn == true)
            {
                throw new System.InvalidOperationException("Can not start twice in a row");
            }
            else
            {
                this._startOn = true;
                this._startTime = DateTime.Now;
                Console.WriteLine("Stopwatch running, type \"stop\" to stop");
                return this._startTime;
            }
        }

        public void Stop()
        {
            this._stopTime = DateTime.Now;
            this._duration = _stopTime - _startTime;
            _StrDuration = _duration.ToString();
            Console.WriteLine(_StrDuration);
            this._startOn = false;
        }
    }
}