C#：foreach與yield語句的介紹

2020-01-24 03:33:38

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1. foreach語句

C#編譯器會把foreach語句轉換為IEnumerable接口的方法和屬性。

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

foreach語句會解析為下面的代碼段。

•調用GetEnumerator（）方法，獲得數組的一個枚舉
•在while循環中，只要MoveNext（）返回true，就一直循環下去
•用Current屬性訪問數組中的元素

復制代碼代碼如下:

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

2. yield語句

•yield語句的兩種形式：

復制代碼代碼如下:

 yield return <expression>;
 yield break;
 

•使用一個yield return語句返回集合的一個元素
•包含yield語句的方法或屬性是迭代器。迭代器必須滿足以下要求
a. 返回類型必須是IEnumerable、IEnumerable<T>、IEnumerator或 IEnumerator<T>。

b. 它不能有任何ref或out參數

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

復制代碼代碼如下:

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語句可以位于try塊或catch塊，但是不能位于finally塊

下面的例子是用yield return語句實現一個簡單集合的代碼，以及用foreach語句迭代集合

復制代碼代碼如下:

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語句返回集合的一個元素，并移動到下一個元素上；yield break可以停止迭代
             yield return "Hello";
             yield return "World";
         }
     }
 }

使用yield return語句實現以不同方式迭代集合的類：

復制代碼代碼如下:

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];
             }
         }
     }
 }