C#中foreach与yield的实例详解【C#.Net教程】,foreach,csharp,yield

1. foreach

C#编译器会把foreach语句转换为IEnumerable接口的要领和属性。

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

foreach语句会剖析为下面的代码段。

挪用GetEnumerator（）要领，取得数组的一个罗列

在while轮回中，只需MoveNext（）返回true，就一向轮回下去

用Current属性接见数组中的元素

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

2. yield语句

yield语句的两种情势：

yield return <expression>;yield break;

运用一个yield return语句返回鸠合的一个元素

包括yield语句的要领或属性是迭代器。迭代器必需满足以下请求

a. 返回范例必需是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语句能够位于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语句能够位于try块或catch块，然则不能位于finally块

下面的例子是用yield return语句完成一个简朴鸠合的代码，以及用foreach语句迭代鸠合

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语句返回鸠合的一个元素，并移动到下一个元素上；yield break能够住手迭代
            yield return "Hello";            yield return "World";
        }
    }
}

运用yield return语句完成以差别体式格局迭代鸠合的类：

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];
            }
        }
    }
}