FROM MSDN Library：IEnumerable、IEnumerator

IEnumerable 介面
-公開能逐一查看非泛型集合內容一次的列舉值。
命名空間： System.Collections
組件： mscorlib (在 mscorlib.dll 中)

IEnumerator 介面
-支援非泛型集合上的簡單反覆運算。
命名空間：   System.Collections
組件：  mscorlib (在 mscorlib.dll 中)

IEnumerator 是所有非泛型列舉值的基底介面。
列舉值可以用來讀取集合中的資料，但是無法用來修改基礎集合。
列舉值一開始會位於集合中第一個元素之前
只要集合保持不變，列舉值就會保持有效。如果已對集合做變更，例如加入、修改或刪除項目，則列舉值將永遠無效

C#
using System;
using System.Collections;

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];
        }
    }

    public IEnumerator 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;
    }

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

class App
{
    static void Main()
    {
        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);

    }
}

/* This code produces output similar to the following:
 * 
 * John Smith
 * Jim Johnson
 * Sue Rabon
 * 
 */

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