一個顯式轉換的問題

[gbcbooks]

namespace Ch11Ex02
{
    public class Animals:CollectionBase
    {
        public Animals this[int animalIndex]
        {
            get
            {
                return (Animal)List[animalIndex];
            }
            set
            {
                List[animalIndex] = value;
            }
        }

        public void Add(Animal newAnimal)
        {
            List.Add(newAnimal);
        }

        public void Remove(Animal newAnimal)
        {
            List.Remove(newAnimal);
        }
    }
}

namespace Ch11Ex02
{
    public class Cow : Animal
    {
        public Cow(string newName):base(newName)
        {
            
        }
    
        public void Milk()
        {
            Console.WriteLine("{0} has been milk.",name);
        }
    }
}

namespace Ch11Ex02
{
    public class Chicken:Animal
    {
        public Chicken(string newName):base(newName)
        {
            
        }
    
        public void LayEgg()
        {
            Console.WriteLine("{0} has laid an egg.",name);
        }
    }
}

namespace Ch11Ex02
{
    public abstract class Animal
    {
        protected string name;
    
        public Animal()
        {
            name = "The animal with no name";
        }

        public Animal(string newName)
        {
            
        }

        public string Name
        {
            get
            {
                return name;
            }
            set
            {
                name = value;
            }
        }

        public void Feed()
        {
            Console.WriteLine("{0} has been fed.",name);
        }
    }
}

namespace Ch11Ex02
{
    class Program
    {
        static void Main(string[] args)
        {
            Animals animalCollection = new Animals();
            animalCollection.Add(new Cow("Jack"));
            animalCollection.Add(new Chicken("Vera"));
            foreach (Animal myAnimal in animalCollection)
            {
                myAnimal.Feed();
            }
            Console.ReadKey();
        }
    }
}

運行時卻提示這一段不能運行

        public Animals this[int animalIndex]
        {
            get
            {
                return (Animal)List[animalIndex];
            }
            set
            {
                List[animalIndex] = value;
            }
        }

不能顯式轉換,這是怎麼一回事?

[gbcbooks]

沒有人來回答???