1、一個Collection要支持foreach方式的遍歷，必須實現IEnumerable接口（亦即，必須以某種方式返回IEnumerator object）。
2、IEnumerator object具體實現了iterator（通過MoveNext()，Reset()，Current）。
3、從這兩個接口的用詞選擇上，也可以看出其不同：IEnumerable是一個聲明式的接口，聲明實現該接口的class是“可枚舉（enumerable）”的，但並沒有說明如何實現枚舉器（iterator）；IEnumerator是一個實現式的接口，IEnumerator object就是一個iterator。
4、IEnumerable和IEnumerator通過IEnumerable的GetEnumerator()方法建立了連接，client可以通過IEnumerable的GetEnumerator()得到IEnumerator object，在這個意義上，將GetEnumerator()看作IEnumerator object的factory method也未嘗不可。
實例程序：
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
*
*/