IEnumerator和IEnumerable区别

IEnumerable接口和IEnumerator接口是.NET中非常重要的接口，二者有何区别？

1. 简单来说IEnumerable是一个声明式的接口，声明实现该接口的类就是“可迭代的enumerable”,但并没用说明如何实现迭代器(iterator).其代码实现为：

public interface IEnumerable

{

IEnumerator GetEnumerator();

}

2. 而IEnumerator接口是实现式接口，它声明实现该接口的类就可以作为一个迭代器iterator.其代码实现为：

public interface IEnumerator

{

object Current { get; }

bool MoveNext();

void Reset();

}

3.一个collection要支持Foreach进行遍历，就必须实现IEnumerable,并一某种方式返回迭代器对象:IEnumerator.

namespace ConsoleApplication1
{
class Program
{
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);
Console.ReadKey();
}
}

public class Person //构造一个Person类
{
public Person(string fName, string lName)
{
this.firstName = fName;
this.lastName = lName;
}

public string firstName;
public string lastName;
}

public class People : IEnumerable // People类是Person的集合类
{
private Person[] _people;
public People(Person[] pArray)//构造函数中将批量Person对象放入集合类的字段中
{
_people = new Person[pArray.Length];

for (int i = 0; i < pArray.Length; i++)
{
_people[i] = pArray[i];
}
}

IEnumerator IEnumerable.GetEnumerator() //实现接口的GetEnumerator方法
{
return (IEnumerator)GetEnumerator();
}

public PeopleEnum GetEnumerator() //重写接口的GetEnumerator方法，自动调用该函数
{
return new PeopleEnum(_people);
}
}

public class PeopleEnum : IEnumerator //实现IEnumerator的类，用来实现迭代
{
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 //IEnumerator接口中的方法必须逐一实现
{
get
{
return Current;
}
}

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

foreach执行的顺序，foreach (Person p in peopleList)，首先程序在peopleList执行public PeopleEnum GetEnumerator()生成一个用来迭代的类。然后执行“in”，在迭代类里面执行public bool MoveNext()，改变迭代类中position的位置。然后跳到执行Person，执行迭代类里面的public Person Current。返回的是当前person对象。