IEnumerable & IEnumerator

IEnumerable 只有一个方法：IEnumerator GetEnumerator(). INumerable 是集合应该实现的一个接口，这样，就能用 foreach 来遍历这个集合。

IEnumerator 有Current属性，MoveNext(), Reset()两个方法。

 

当 foreach 使用到一个 IEnumerable 的集合上的时候，遍历是这样开始的：

1. 调用 GetEnumerator() 得到一个 IEnumerator 的对象。

2. 调用 MoveNext();

3. 使用 其中一个对象。

4. 重复2和3, 直到 MoveNext（）返回 false（没有下一个啦）.

 

由此可见

1. GetEnumerator() 返回的对象在最开始的时候，指针是放在第一个对象之前的，Reset（）之后也是这样。

2. 为了使用 foreach， 实现 IEnumerable 不是必须的，只是一个 best practice 而已。

 

下面我们来看微软提供的一个例子： 

using System;
using System.Collections;

// Simple business object.
public class Person
{
    public Person(string fName, string lName)
    {
        this.firstName = fName;
        this.lastName = lName;
    }

    public string firstName;
    public string lastName;
}

// Collection of Person objects. This class
// implements IEnumerable so that it can be used
// with ForEach syntax.
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];
        }
    }

// Implementation for the GetEnumerator method.
    IEnumerator IEnumerable.GetEnumerator()
    {
       return (IEnumerator) GetEnumerator();
    }

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

// When you implement IEnumerable, you must also implement IEnumerator.
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 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
 *
 */