关于迭代器及yield的用法

1、foreach语句

C#编译器会把foreach语句转换为IEnumerable接口的方法和属性。

 foreach (Person p in persons)
 {
 Console.WriteLine(p);
 }

foreach语句会解析为下面的代码段。

• 调用GetEnumerator（）方法，获得数组的一个枚举
• 在while循环中，只要MoveNext（）返回true，就一直循环下去
• 用Current属性访问数组中的元素

 IEnumerator enumerator = persons. GetEnumerator();
 while (enumerator.MoveNext())
 {
 Person p = (Person) enumerator.Current;
 Console.WriteLine(p);
 }

2. yield语句

• yield语句的两种形式：

 yield return <expression>;
 yield break;

• 使用一个yield return语句返回集合的一个元素
• 包含yield语句的方法或属性是迭代器。迭代器必须满足以下要求

a. 返回类型必须是IEnumerable、IEnumerable<T>、IEnumerator或 IEnumerator<T>。

b. 它不能有任何ref或out参数

• yield return语句不能位于try-catch快。yield return语句可以位于try-finally的try块

             try
             {
                 // ERROR: Cannot yield a value in the boday of a try block with a catch clause
                 yield return "test";
             }
             catch
             { }
 
             try
             {
                 //
                 yield return "test again";
             }
             finally
             { }
 
             try
             { }
             finally
             {
                 // ERROR: Cannot yield in the body of a finally clause
                 yield return "";
             }

• yield break语句可以位于try块或catch块，但是不能位于finally块

3、实例

class Program
    {
        static void Main(string[] args)
        {
            foreach (int i in List.Power(,))
            {
                Console.WriteLine(i);
            }
        }
    }
 
    public class List
    {
        public static IEnumerable<int> Power(int num, int exponent)
        {
            int counter = ;
            int result = ;
            while (counter++ < exponent)
            {
                result *= num;
                yield return result;
                Thread.Sleep();//效果很有意思
            }
        }
    }

 internal class Program
    {
         static void Main(string[] args)
        {
            A a=new A();
            foreach (string s in a)
            {
                Console.WriteLine(s);
            }
             Console.WriteLine();
 
             foreach (string s in a.Reverse())
             {
                 Console.WriteLine(s);
             }
             Console.WriteLine();
 
             foreach (string s in a.GetEnumerator(,))
             {
                 Console.WriteLine(s);
             }
             Console.ReadKey();
        }
    }
 
    public class A
    {
        private string[] strs = {"a", "b", "c", "d"};
 
        public IEnumerator<string> GetEnumerator()
        {
            for (int i = ; i < ; i++)
            {
                yield return strs[i];
            }
        }
 
        public IEnumerable<string> Reverse()
        {
            for (int i = ; i>=; i--)
            {
                yield return strs[i];
            }
        }
 
        public IEnumerable<string> GetEnumerator(int index,int length)
        {
            for (int i = index; i < index+length ; i++)
            {
                yield return strs[i];
            }
        }
    }

结果：

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" 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