C#使用yield关键字构建迭代器详解

以前，如果我们希望构建支持foreach枚举的自定义集合，只能实现ienumerable接口（可能还有ienumerator()）,返回值还必须是ienumerator类型，除此之外还可以通过迭代器来使用构建foreach循环的类型，详细见下链接。

 代码

 public class car
  {
    //内部状态数据
    public int curentspeed;
    public int maxspeed;
    public string name;
    //汽车能不能用
    private bool carisdead;
    //类构造函数
    public car() { }
    public car(string name, int currentspeed, int maxspeed = 100)
    {
      this.name = name;
      this.curentspeed = currentspeed;
      this.maxspeed = maxspeed;
    }
    //定义委托类型
    public delegate void carenginehandler(string msdforcar);
    //定义每个委托类型的成员变量
    private carenginehandler listofhandlers;
    //向调用者添加注册函数
    public void registerwithcarengine(carenginehandler methodtocall)
    {
      if (listofhandlers == null)
        listofhandlers = methodtocall;
      else
        listofhandlers += methodtocall;//支持多路广播
    }
    //实现accelerate()方法
    public void accelerate(int delta)
    {
      if (carisdead)
      {
        if (listofhandlers != null)
        {
          listofhandlers("sorry,this car is dead");
        }
      }
      else
      {
        curentspeed += delta;
        //不能超过最大速度
        if (5 == (maxspeed - curentspeed) && listofhandlers != null)
        {
          listofhandlers("this speed is nearly to the maxspeed");
        }
        if (curentspeed > maxspeed)
        {
          carisdead = true;
        }
        else
          console.writeline("current speed:{0}", curentspeed);
      }
    }
  }
  public class garage : ienumerable
  {
    private car[] garage = new car[3];
    public garage()
    {
      garage[0] = new car("a", 10);
      garage[1] = new car("b", 13);
      garage[2] = new car("c", 14);
    }
    public enumerator getenumerator()
    {
      //返回数组对象的ienumerator
      //return garage.getenumerator();
      //用yield关键字构建迭代器方法
      foreach (car c in garage)
      {
        //当yield return语句执行后，当前位会被
        //保存下来，下一次执行会从当前位开始
        yield return c;
      }
    }
  }
  class program
  {
    static void main(string[] args)
    {
      garage g = new garage();
      foreach (car c in g)
      {
        console.writeline("car name:{0}", c.name);
      }
    }
  }

参考：c#中可枚举类型详解

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