6.2 C# 2：利用 yield 语句简化迭代器

 1     class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             object[] values = new object[] { "a", "b", "c", "d", "e" };
 6             IterationSample sample = new IterationSample(values, 3);
 7             foreach (var item in sample)
 8             {
 9                 Console.WriteLine(item);
10             }
11             Console.ReadKey();
12         }
13     }
14     public class IterationSample : IEnumerable
15     {
16         public object[] values;
17         int startPoint;
18         public IterationSample(object[] values, int startingPoint)
19         {
20             this.values = values;
21             this.startPoint = startingPoint;
22         }
23         public IEnumerator GetEnumerator()
24         {
25             //return new IterationSampleIterator(this);
26             for (int index = 0; index )
27             {
28                 yield return values[(index + startPoint) % values.Length];
29             } 
30         }
31     }
32     public class IterationSampleIterator : IEnumerator
33     {
34         IterationSample parent;
35         int position;
36         public IterationSampleIterator(IterationSample parent)
37         {
38             this.parent = parent;
39             this.position = -1;
40         }
41         public object Current
42         {
43             get
44             {
45                 if (position == -1 || position == parent.values.Length)
46                 {
47                     throw new InvalidOperationException();
48                 }
49                 int index = position + parent.values.Length;
50                 index = index % parent.values.Length;
51                 return parent.values[index];
52             }
53         }
54 
55         public bool MoveNext()
56         {
57             if (position != parent.values.Length)
58             {
59                 position++;
60             }
61             return position  parent.values.Length;
62         }
63 
64         public void Reset()
65         {
66             position = -1;
67         }
68     }

 1     class Program
 2     {
 3         static readonly string Padding = new string(‘ ‘, 30);
 4         static void Main(string[] args)
 5         {
 6             IEnumerableint> iterable = CreatteEnumerable(Padding);
 7             IEnumeratorint> iterator = iterable.GetEnumerator();
 8             Console.WriteLine("starting iterate");
 9 
10             while (true)
11             {
12                 Console.WriteLine("calling MoveNext()");
13                 bool result = iterator.MoveNext();
14                 Console.WriteLine("moveNext result={0}", result);
15                 if (!result)
16                     break;
17                 Console.WriteLine("fetching current");
18                 Console.WriteLine("current result ={0}", iterator.Current);
19             }
20 
21             Console.ReadKey();
22         }
23         static IEnumerableint> CreatteEnumerable(string Padding)
24         {
25             Console.WriteLine("{0} start of createEnumerbale padding", Padding);
26 
27             for (int i = 0; i 3; i++)
28             {
29                 Console.WriteLine("{0} about to yield {1}", Padding, i);
30                 yield return i;
31                 Console.WriteLine("{0} after padding", Padding);
32             }
33             Console.WriteLine("{0} yield final value ", Padding);
34             yield return -1;
35             Console.WriteLine("{0} end of createEnumerable();", Padding);
36         }
37         /*
38         
39         starting iterate
40         calling MoveNext()
41                                        start of createEnumerbale padding
42                                        about to yield 0
43         moveNext result=True
44         fetching current
45         current result =0
46         calling MoveNext()
47                                        after padding
48                                        about to yield 1
49         moveNext result=True
50         fetching current
51         current result =1
52         calling MoveNext()
53                                        after padding
54                                        about to yield 2
55         moveNext result=True
56         fetching current
57         current result =2
58         calling MoveNext()
59                                        after padding
60                                        yield final value
61         moveNext result=True
62         fetching current
63         current result =-1
64         calling MoveNext()
65                                        end of createEnumerable();
66         moveNext result=False
67 
68          */
69     }