public static void TestCars4()
        {
            System.Console.WriteLine("\nTestCars4");
            System.Console.WriteLine("----------");
            Car car2 = new ConvertibleCar();
            Car car3 = new Minivan();

            car2.ShowDetails();
            car3.ShowDetails();
        }
        public static void TestCars1()
        {
            System.Console.WriteLine("\nTestCars1");
            System.Console.WriteLine("----------");

            Car car1 = new Car();

            car1.DescribeCar();
            System.Console.WriteLine("----------");

            // Notice the output from this test case. The new modifier is
            // used in the definition of ShowDetails in the ConvertibleCar
            // class.

            ConvertibleCar car2 = new ConvertibleCar();

            car2.DescribeCar();
            System.Console.WriteLine("----------");

            Minivan car3 = new Minivan();

            car3.DescribeCar();
            System.Console.WriteLine("----------");
        }