Implement IEnumerable

Program.cs

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

namespace CreateAndImplClassHierar
{
    class Program
    {
        static void Main(string[] args)
        {
            var race = new Race("Crash Team Racing");

            var car1 = new RaceCar(1, "Doctor Neo Cortex");
            var car2 = new RaceCar(2, "Crash Bandicoot");
            var car3 = new RaceCar(2, "Doctor N. Gin");

            race.AddCar(car1);
            race.AddCar(car2);
            race.AddCar(car3);

            foreach (var car in race)
            {
                Console.WriteLine($"Car number: {car.Number}, Driver: {car.Driver}");
            }

            Console.ReadKey();
        }
    }

    public class Race : IEnumerable<RaceCar>
    {
        private List<RaceCar> _cars;

        public Race(string title)
        {
            Title = title;
            _cars = new List<RaceCar>();
        }

        public string Title { get; set; }

        public void AddCar(RaceCar car)
        {
            _cars.Add(car);
        }

        public IEnumerator<RaceCar> GetEnumerator()
        {
            foreach(var car in _cars)
            {
                yield return car;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

    public class RaceCar
    {
        public RaceCar(int number, string driver)
        {
            Number = number;
            Driver = driver;
        }

        public int Number { get; set; }
        public string Driver { get; set; }
    }
}