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：

aaarticlea/png;base64,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" alt="" />