C# foreach 原理

foreach原理

IEnumerator GetEnumerator()在using System.Collections;命名空间。

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		BookShelf bs = new BookShelf()
		{
		   AllBooks = new IRead[]
		   {
		       new Book1(),
			   new Book2(),
			   new Book3()
		   }
		};
		foreach(var item in bs)
		{
		   Console.WriteLine(item);
		}
		Console.WriteLine("END");
	}
}
//原理
public class BookShelf //:IEnumerable
{
     public IRead[] AllBooks{get;set;}
	 public  IEnumerator GetEnumerator()
	 {
	     //throw new NotImplementedException();
		 return new MyIEnumerator(){ Target = AllBooks };
	 }
}

//迭代器
public class MyIEnumerator:IEnumerator
{
   public IRead[] Target{get;set;}
   public int index=-1;
   //2.获取当前元素
   public object Current{get{return Target[index];}}
   //1.先移动到下一元素,返回一个bool值。
   public bool MoveNext()
   {
      index++;
      return index<=Target.Length-1;
   }
   public void Reset(){}
}



public class Book1:IRead
{
    public  void Read()
	{
	   Console.WriteLine("C++");
	}
}

public class Book2:IRead
{
    public  void Read()
	{
	   Console.WriteLine("C#");
	}
}

public class Book3:IRead
{
    public  void Read()
	{
	   Console.WriteLine("python");
	}
}


public interface IRead
{
    void Read();
}

运行结果：

Book1
Book2
Book3
END

--------------------------------------------------------------------------------------------------------------------------

简化代码

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		BookShelf bs = new BookShelf()
		{
		   AllBooks = new IRead[]
		   {
		       new Book1(),
			   new Book2(),
			   new Book3()
		   }
		};
		foreach(var item in bs)
		{
		   Console.WriteLine(item);
		}
		Console.WriteLine("END");
	}
}
//原理 yield 
public class BookShelf //:IEnumerable
{
     public IRead[] AllBooks{get;set;}
	 public  IEnumerator GetEnumerator()
	 {
	    for(int i =0;i<AllBooks.Length;i++)
		{
		   yield return AllBooks[i];
		}
	 }
}

public class Book1:IRead
{
    public  void Read()
	{
	   Console.WriteLine("C++");
	}
}

public class Book2:IRead
{
    public  void Read()
	{
	   Console.WriteLine("C#");
	}
}

public class Book3:IRead
{
    public void Read()
	{
	   Console.WriteLine("python");
	}
}


public interface IRead
{
    void Read();
}

Book1
Book2
Book3
END