IEnumerable是所有可迭代非范型类的基础接口。IEnumerable包括一个方法GetEnumerator方法，方法返回一个IEnumerator。

IEnumerator是所有非范型迭代器的基础接口。foreach语句隐藏了C#迭代器的复杂实现。推荐使用foreach代替直接操作迭代器。
迭代器可以读取集合中的数据，但是不能从底层修改集合。
初始的时候，迭代器定位在集合的第一个元素前面，在读取Current值之前需要调用一次MoveNext将迭代器驱动到第一个元素的位置。
Current一直返回相同的元素直到调用了MoveNext或者Reset方法。MoveNext将Current推进到下一个元素。
如果MoveNext之后position超出了集合的范围，MoveNext将返回false。
通过调用Reset将Current重置到第一个元素之前。

于是我们不难得出

class Program
    {
        static void Main(string[] args)
        {
            Person[] peopleArray = new Person[3]
            {
                new Person("John","Smith"),
                new Person("Tom","Johnson"),
                new Person("Sue","Robon"),
            };
            People peopleList = new People(peopleArray);
            foreach (Person person in peopleList)
            {
                Console.WriteLine(person.firstName + person.lastName);
            }

            IEnumerator enumerator = peopleList.GetEnumerator();
            while (enumerator.MoveNext())
            {
                Person person = (Person)enumerator.Current;
                Console.WriteLine(person.firstName + person.lastName);
            }
        }
    }

上面两种写法是一样的，foreach是一种语法糖，简化了遍历其中的具体实现。被遍历的类通过实现IEnumerable接口和实现一个IEnumerator枚举器实现遍历功能。

使用实例：

class Program
    {
        static void Main(string[] args)
        {
            Person[] peopleArray = new Person[3]
            {
                new Person("John","Smith"),
                new Person("Tom","Johnson"),
                new Person("Sue","Robon"),
            };
            People peopleList = new People(peopleArray);
            foreach (Person person in peopleList)
            {
                Console.WriteLine(person.firstName + person.lastName);
            }
        }
    }
    public class Person
    {
        public string firstName;
        public string lastName;

        public Person(string firstName, string lastName)
        {
            this.firstName = firstName;
            this.lastName = lastName;
        }
    }

    public class People : IEnumerable
    {
        private Person[] _people;
        public People(Person[] pArray)
        {
            _people = new Person[pArray.Length];
            for (int i = 0; i < pArray.Length; i++)
            {
                _people[i] = pArray[i];
            }
        }

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

        public PeopleEnum GetEnumerator()
        {
            return new PeopleEnum(_people);
        }
    }

    public class PeopleEnum:IEnumerator
    {
        public Person[] _people;
        int position = -1;
        public PeopleEnum(Person[] list)
        {
            _people = list;
        }

        Object IEnumerator.Current
        {
            get
            {
                return Current;
            }
        }

        public Person Current
        {
            get
            {
                try
                {
                    return _people[position];
                }
                catch(IndexOutOfRangeException)
                {
                    throw new InvalidOperationException();
                }
            }
        }

        public bool MoveNext()
        {
            position++;
            return (position < _people.Length);
        }

        public void Reset()
        {
            position = -1;
        }
    }