C#：foreach与yield语句的介绍

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
 

下面的例子是用yield return语句实现一个简单集合的代码，以及用foreach语句迭代集合

using system;
 using system.collections.generic;

 namespace consoleapplication6
 {
     class program
     {
         static void main(string[] args)
         {
             hellocollection hellocollection = new hellocollection();
             foreach (string s in hellocollection)
             {
                 console.writeline(s);
                 console.readline();
             }
         }
     }

     public class hellocollection
     {

         public ienumerator<string> getenumerator()
         {
             // yield return语句返回集合的一个元素，并移动到下一个元素上；yield break可以停止迭代
             yield return "hello";
             yield return "world";
         }
     }
 }

使用yield return语句实现以不同方式迭代集合的类：

using system;
 using system.collections.generic;

 namespace consoleapplication8
 {
     class program
     {
         static void main(string[] args)
         {
             musictitles titles = new musictitles();
             foreach (string title in titles)
             {
                 console.writeline(title);
             }
             console.writeline();

             foreach (string title in titles.reverse())
             {
                 console.writeline(title);
             }
             console.writeline();

             foreach (string title in titles.subset(2, 2))
             {
                 console.writeline(title);
                 console.readline();
             }
         }
     }

     public class musictitles
     {
         string[] names = { "a", "b", "c", "d" };
         public ienumerator<string> getenumerator()
         {
             for (int i = 0; i < 4; i++)
             {
                 yield return names[i];
             }
         }

         public ienumerable<string> reverse()
         {
             for (int i = 3; i >= 0; i--)
             {
                 yield return names[i];
             }
         }

         public ienumerable<string> subset(int index, int length)
         {
             for (int i = index; i < index + length; i++)
             {
                 yield return names[i];
             }
         }
     }
 }

输出：