.NET迭代器

时间：2011-02-24 来源：雁北飞

如MSDN上面的代码：

View Code

 1 using System;
 2  using System.Collections;
 3 
 4  public class Person
 5 {
 6     public Person(string fName, string lName)
 7     {
 8         this.firstName = fName;
 9         this.lastName = lName;
10     }
11 
12     public string firstName;
13     public string lastName;
14 }
15 
16 public class People : IEnumerable
17 {
18     private Person[] _people;
19     public People(Person[] pArray)
20     {
21         _people = new Person[pArray.Length];
22 
23         for (int i = 0; i < pArray.Length; i++)
24         {
25             _people[i] = pArray[i];
26         }
27     }
28 
29     public IEnumerator GetEnumerator()
30     {
31         return new PeopleEnum(_people);
32     }
33 }
34 
35 public class PeopleEnum : IEnumerator
36 {
37     public Person[] _people;
38 
39     // Enumerators are positioned before the first element
40     // until the first MoveNext() call.
41     int position = -1;
42 
43     public PeopleEnum(Person[] list)
44     {
45         _people = list;
46     }
47 
48     public bool MoveNext()
49     {
50         position++;
51         return (position < _people.Length);
52     }
53 
54     public void Reset()
55     {
56         position = -1;
57     }
58 
59     public object Current
60     {
61         get
62         {
63             try
64             {
65                 return _people[position];
66             }
67             catch (IndexOutOfRangeException)
68             {
69                 throw new InvalidOperationException();
70             }
71         }
72     }
73 }
74 
75 class App
76 {
77     static void Main()
78     {
79         Person[] peopleArray = new Person[3]
80         {
81             new Person("John", "Smith"),
82             new Person("Jim", "Johnson"),
83             new Person("Sue", "Rabon"),
84         };
85 
86         People peopleList = new People(peopleArray);
87         foreach (Person p in peopleList)
88             Console.WriteLine(p.firstName + " " + p.lastName);
89 
90     }
91 }
92 
93 /* This code produces output similar to the following:
94  * 
95  * John Smith
96  * Jim Johnson
97  * Sue Rabon
98  * 
99  */

这样的实现想必是有些麻烦。迭代器就是让我们的实现foreach的功能更加简便。先来看看MSDN上面关于迭代器的定义。

迭代器是定义在结构或者类中，其定义形式可以是方法，get访问器，运算符。要求定义后的迭代器的返回类型必须是IEnumerable、IEnumerator、IEnumerable<T> 或 IEnumerator<T>。这四者之中的一个。通过定义迭代器，我们就可以实现对类，结构中的数据结构实现访问。而不必像上面一样，完全实现IEnumerable、IEnumerator接口。当编译器检测到迭代器时，它将自动生成 IEnumerable 或 IEnumerable<T> 接口的 Current、MoveNext 和 Dispose 方法。

需要注意一点的是，在迭代器中引入了一个新的关键字：yield。迭代器代码使用 yield return 语句依次返回每个元素。yield break 将终止迭代。

下面是通过使用迭代器实现上面功能的代码：

方式1：

没有继承自IEnumerable

View Code

 1 public class People     {
 2         private Person[] _people;
 3         public People(Person[] pArray)
 4         {
 5             _people = new Person[pArray.Length];
 6 
 7             for (int i = 0; i < pArray.Length; i++)
 8             {
 9                 _people[i] = pArray[i];
10             }
11         }
12 
13         public IEnumerable Test()
14         {
15             for (int i = 0; i < _people.Length; i++)
16             {
17                 yield return _people[i];
18             }
19         }
20 
21             }

foreach访问方式：

View Code

1 foreach (Person item in people.Test())
2    {
3     Console.WriteLine(item.firstName);
4    }

实现方式2：

继承自IEnumerable。

foreach访问方式：

View Code

 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         public IEnumerable Test()
15         {
16             for (int i = 0; i < _people.Length; i++)
17             {
18                 yield return _people[i];
19             }
20         }
21 
22         public IEnumerator GetEnumerator()
23         {
24             for (int i = 0; i < _people.Length; i++)
25             {
26                 yield return _people[i];
27             }
28         }
29     }

View Code

1 foreach (Person item in people)
2             {
3                 Console.WriteLine(item.firstName);
4             }