using System;
using System.Collections;

namespace CS2026
{
    public delegate void SpeedChangedEventHandler(object source, SpeedChangedEventArgs e);

    public class SpeedChangedEventArgs : EventArgs
    {
        int _oldSpeed;
        int _newSpeed;

        public int OldSpeed
        {
            get
            {
                return _oldSpeed;
            }
            set
            {
                _oldSpeed = value;
            }
        }

        public int NewSpeed
        {
            get
            {
                return _newSpeed;
            }
            set
            {
                _newSpeed = value;
            }
        }
    }

    public class Car
    {
        SpeedChangedEventHandler _speedChangedEventHandler;
        object lockThis = new object();
        int _speed = 0;

        public event SpeedChangedEventHandler SpeedChanged
        {
            add
            {
                lock (lockThis)
                {
                    _speedChangedEventHandler += value;
                }
            }
            remove
            {
                lock (lockThis)
                {
                    _speedChangedEventHandler -= value;
                }
            }
        }

        public virtual void OnSpeedChanged(int oldSpeed, int newSpeed)
        {
            SpeedChangedEventArgs e = new SpeedChangedEventArgs();
            e.OldSpeed = oldSpeed;
            e.NewSpeed = newSpeed;

            if (_speedChangedEventHandler != null)
                _speedChangedEventHandler(this, e);
        }

        public int Speed
        {
            get
            {
                return _speed;
            }
            set
            {
                int oldSpeed = _speed;
                _speed = value;
                OnSpeedChanged(oldSpeed, _speed);
            }
        }

        static void Main(string[] args)
        {
            Car car = new Car();
            car.Speed = 25;
            car.SpeedChanged += new SpeedChangedEventHandler(CarSpeedChanged);
            car.Speed = 50;
            car.SpeedChanged -= new SpeedChangedEventHandler(CarSpeedChanged);
            car.Speed = 75;
        }

        static void CarSpeedChanged(object source, SpeedChangedEventArgs e)
        {
            Console.WriteLine("Car speed changed from {0} to {1}",
            e.OldSpeed, e.NewSpeed);
        }

    }

}