ChocolateBoiler.cs

using System;

namespace SingletonPattern
{
    internal partial class ChocolateBoiler
    {
        private static readonly Lazy<ChocolateBoiler> _singleton = new Lazy<ChocolateBoiler>(() => new ChocolateBoiler());
        
        public static ChocolateBoiler GetInstance() => _singleton.Value;
      
        private Status _boiler;

        private ChocolateBoiler()
        {
            Console.WriteLine("Starting");
            _boiler = Status.Empty;
        }

        public void Fill()
        {
            if (!IsEmpty) return;
            Console.WriteLine("Filling...");
            _boiler = Status.InProgress;
        }

        public void Drain()
        {
            if (!IsBoiled) return;
            Console.WriteLine("Draining...");
            _boiler = Status.Empty;
        }

        public void Boil()
        {
            if (IsBoiled || IsEmpty) return;
            Console.WriteLine("Boiling...");
            _boiler = Status.Boiled;
        }

        private bool IsEmpty => (_boiler == Status.Empty);

        private bool IsBoiled => (_boiler == Status.Boiled);
    }
}