IEnumerator,IEnumerable接口,yield关键字

foreach可以迭代一些.net本来就为我们做好的类型，比如int,string等等

但是如果想迭代自己的类，就必须自己来写了

首先就要先实现IEnumerator的几个方法，和IEnumerable的GetEnumerator（）方法

 

代码

foreach (person p  in  persons)
{
   console.writeline(p)
}
//会解析为
IEnumerator enumerator  =  persons.GetEnumerator();
while (enumerator.MoveNext())
{
   Person p  =  (Person)enumerator.Current;
   Console.WriteLine(p);
}

 

 

也就是说，foreach循环会调用IEnumerator下的几个方法

他们分别是：public bool MoveNext()，public void Reset()， public object Current属性

顾名思义

 

代码

   public   class  Person
    {
         public  Person( string  fName,  string  lName)
        {
             this .firstName  =  fName;
             this .lastName  =  lName;
        }

         public   string  firstName;
         public   string  lastName;
    }

     public   class  People : IEnumerable
    {
         private  Person[] _people;
         public  People(Person[] pArray)
        {
             // _people=pArray?????????
            _people  =   new  Person[pArray.Length];

             for  ( int  i  =   0 ; i  <  pArray.Length; i ++ )
            {
                _people[i]  =  pArray[i];
            }
        }
         ///   <summary>
         ///  接口的实现，返回的是一个PeopleEnum的一个类对象
         ///   </summary>
         ///   <returns></returns>
         public  IEnumerator GetEnumerator()
        {
             return   new  PeopleEnum(_people);
        }
    }

     public   class  PeopleEnum : IEnumerator
    {
         public  Person[] _people;

         //  Enumerators are positioned before the first element
         //  until the first MoveNext() call.
         int  position  =   - 1 ;

         public  PeopleEnum(Person[] list)
        {
            _people  =  list;
        }

         public   bool  MoveNext()
        {
            position ++ ;
             return  (position  <  _people.Length);
        }

         public   void  Reset()
        {
            position  =   - 1 ;
        }

         public   object  Current
        {
             get
            {
                 try
                {
                     return  _people[position];
                }
                 catch  (IndexOutOfRangeException)
                {
                     throw   new  InvalidOperationException();
                }
            }
        }
    }

     class  App
    {
         static   void  Main()
        {
            Person[] peopleArray  =   new  Person[ 3 ]
            {
                new  Person( " John " ,  " Smith " ),
                new  Person( " Jim " ,  " Johnson " ),
                new  Person( " Sue " ,  " Rabon " ),
            };

            People peopleList  =   new  People(peopleArray);
             foreach  (Person p  in  peopleList)
            {
                Console.WriteLine(p.firstName  +   "   "   +  p.lastName);
            }
            Console.Read();
        }
    }

 

总结： 迭代自定义的类，必须实现实现GetEnumerator()方法返回IEnumerator

IEnumerator提供了foreach使用的方法属性

 

 

 

C#2.0有了yield关键字，就不用每次都这样实现IEnumerator接口了

包含yield语句的方法必须返回IEnumerator或者IEnumerable接口

 

代码

    public   class  person
    {
         private   string  _Name;
         public   string  Name {  get ;  set ; } 
         public  person( string  name)
        {
            Name  =  name;
        }
         public   override   string  ToString()
        {
             return   this .Name;
        }
    }
     class  people
    {
         private  person[] _person;
         public  people(person[] person)
        {
            _person  =  person;
        }
         public  IEnumerator GetEnumerator()
        {
             for  ( int  i  =   0 ; i  <  _person.Length; i ++ )
            {
                 yield   return  _person[i];
            }
        }
         public  IEnumerable reverse()
        {
             for  ( int  i  =  _person.Length  -   1 ; i  >=   0 ; i -- )
            {
                 yield   return  _person[i];
            }
        }
    }
     class  Program
    {
         static   void  Main( string [] args)
        {
            person[] p  =   new  person[ 3 ]
            {
                new  person( " John " ),
                new  person( " Jim " ),
                new  person( " Sue " ),
            };
            people pp  =   new  people(p);
             foreach  (person pe  in  pp)
            {
                Console.WriteLine(pe);
            }
             foreach  (person ppp  in  pp.reverse())
            {
                Console.WriteLine(ppp);
            }

            Console.Read();
        }
    }

 

 

我认为yield关键字帮助我们实现了IEnumerator的方法

也可以自己定义迭代的方式，比如以上代码就实现了逆向循环。

 

参考：C#红皮书