请各位大大帮忙解决下一个泛型类的问题

Question

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;namespace Ch12Ex04
{
    public class Farm<T> : IEnumerable<T>
        where T : Animal
    {
        private List<T> animals = new List<T>();        public List<T> Animals
        {
            get
            {
                return animals;
            }
        }
        //实现IEnumerable接口
        public IEnumerable<T> GetEnumrator()
        {
            return animals.GetEnumerator();
        }
        
        IEnumerator IEnumerable.GetEnumerator()
        {
            return animals.GetEnumerator();
        }
        //迭代两个基类方法
        public void MakeNoises()
        {
            foreach (T animal in animals)
            {
                animal.MakeANoise();
            }
        }
        public void FeedTheAnimals()
        {
            foreach (T animal in animals)
            {
                animal.Feed();
            }
        }
        //得到所有Cow或者继承于其的类
        public Farm<Cow> GetCows()
        {
            Farm<Cow> cowFarm = new Farm<Cow>();
            foreach (T animal in animals)
            {
                if (animal is Cow)
                {
                    cowFarm.Animals.Add(animal as Cow);
                }
            }
            return cowFarm;
        }
    }
}
这是<<C#入门经典>>里的一例题中的一个泛型类Farm.cs，用已有类Animal来约束。
为何我在编译运行时出现了错误“Ch12Ex04.Farm<T>”不会实现接口成员“System.Collections.Generic.IEnumerable<T>.GetEnumerator()” E:\Chapter12\Ch12Ex04\Ch12Ex04\Farm.cs？？请各位帮忙解答下，谢谢

Answer 1

IEnumerator IEnumerable.GetEnumerator()
=========================================
public IEnumerator<T> GetEnumerator()

Answer 2

源码是一个Animal.cs类，两个继承于其的Cow，Chicken类，和一个继承于Cow的SuperCow类
using System;
using System.Collections.Generic;
using System.Text;
using System.Collections;namespace Ch12Ex04
{
    public abstract class Animal
    {
        protected string name;
        public string Name
        {
            get
            {
                return name;
            }
            set
            {
                name = value;
            }
        }
        //构造函数2
        public Animal()
        {
            name = "The Animal with no name.";
        }
        public Animal(string newName)
        {
            name = newName;
        }
        public void Feed()
        {
            Console.WriteLine("{0} has been fed.", name);
        }
        //抽象方法
        public abstract void MakeANoise();
    }
}

Answer 3

Cow类
using System;
using System.Collections.Generic;
using System.Text;namespace Ch12Ex04
{
    public class Cow:Animal
    {
        public void Milk()
        {
            Console.WriteLine("{0} has been milked.",name);
        }
        public Cow(string newName):base(newName)
        {
        }
        
        public override void MakeANoise()
        {
            Console.WriteLine("{0} says 'moo'", name);
        }    }
}

Answer 4

Chick.cs类
using System;
using System.Collections.Generic;
using System.Text;namespace Ch12Ex04
{
    public class Chicken:Animal
    {
        public void LayEgg()
        {
            Console.WriteLine("{0} has laid an egg.", name);
        }
        public Chicken(string newName):base(newName)
        {
        }
        public override void MakeANoise()
        {
            Console.WriteLine("{0} says '咯咯'", name);
        }    }
}