实现foreach遍历

【实现foreach遍历】

　　IEnumerable的原始版本存在于System.Collection中。

　　一个类想要被foreach遍历，需要实现此IEnumerable接口。

 1 public class People : IEnumerable
 2 {
 3     private Person[] _people;
 4     public People(Person[] pArray)
 5     {
 6         _people = new Person[pArray.Length];
 7 
 8         for (int i = 0; i < pArray.Length; i++)
 9         {
10             _people[i] = pArray[i];
11         }
12     }
13 
14     IEnumerator IEnumerable.GetEnumerator()
15     {
16        return (IEnumerator) GetEnumerator();
17     }
18 
19     public PeopleEnum GetEnumerator()
20     {
21         return new PeopleEnum(_people);
22     }
23 }
24 
25 class App
26 {
27     static void Main()
28     {
29         Person[] peopleArray = new Person[3]
30         {
31             new Person("John", "Smith"),
32             new Person("Jim", "Johnson"),
33             new Person("Sue", "Rabon"),
34         };
35 
36         People peopleList = new People(peopleArray);
37         foreach (Person p in peopleList)
38             Console.WriteLine(p.firstName + " " + p.lastName);
39 
40     }
41 }

View Code

　　为了遍历能真正成功，需要返回一个实现了IEnumerator的迭代器。

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();
            }
        }
    }
}

View Code

　　IEnumerator原型如下：

【泛型版本】

　　IEnumerator默认返回的是object，为了适应多种不同的情形，微软为IEnumerator提供了一个泛型版本。

　　上面看到，IEnumerator是由IEnumerable返回，显然IEnumerable必须提供这个T。所以IEnuemrable也有相应的泛型版本。

参考：http://msdn.microsoft.com/zh-cn/library/9eekhta0(v=vs.110).aspx