实现自定义集合的可枚举类型（IEnumerable）和枚举数（IEnumerator ）

下面的代码示例演示如何实现自定义集合的 IEnumerable 和 IEnumerator 接口：

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    public class Person
    {
        public Person(string fName, string lName)
        {
            this.firstName = fName;
            this.lastName = lName;
        }

        public string firstName;
        public string 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;

        // Enumerators are positioned before the first element
        // until the first MoveNext() call.
        int position = -1;

        public PeopleEnum(Person[] list)
        {
            _people = list;
        }

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

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

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

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


    class Program
    {
        static void Main(string[] args)
        {
            Person[] peopleArray = new Person[3]
            {
                new Person("John", "Smith"),
                new Person("Jim", "Johnson"),
                new Person("Sue", "Rabon"),
            };

            People peopleList = new People(peopleArray);
            foreach (Person p in peopleList)
                Console.WriteLine(p.firstName + " " + p.lastName);
        }
    }
}

可枚举类型 → 实现IEnumerable接口，可以不需要直接实现这个接口，但必须有个GetEnumerator方法，返回值类型必须为IEnumerator类型，也就是第四点最后一段代码中接口注释的那种写法！

枚举数 → 实现IEnumerator接口，实现全部方法，首先是调用GetEnumerator返回一个类型为IEnumerator的枚举数，然后编译器会隐式的调用实现IEnumerator类中的方法和属性！

总结：所以实现foreach遍历，必须达到上面的两种条件才能进行遍历对象，他们可以写在一起也可以分开，最好是分开，进行职责分离，一个类干一件事总归是好事！也满足面向对象的单一指责设计原则。