C# Stopwatch Class

Stopwatch.cs

using System;

namespace Sandbox
{
    public class Stopwatch
    {
        private bool _started;
        private DateTime _startTime;
        public TimeSpan Duration { get; private set; }
        
        public void Start()
        {
            if (_started) throw new InvalidOperationException("Stopwatch already started.");
            _started = true;
            _startTime = DateTime.Now;
        }

        public void Stop()
        {
            if (!_started) throw new InvalidOperationException("Stopwatch not started.");
           
            this.Duration += (DateTime.Now - _startTime);
            _started = false;
            
        }

        public void Reset()
        {
            _started = false;
            this.Duration = TimeSpan.Zero;
        }
    }
}