C# IEnumerable 和 IEnumerator

IEnumerable 和 IEnumerator区别

这段时间写点基础文章，这次写的是C# IEnumerable 和 IEnumerator，这两者的区别在于：

• IEnumerable其意义就是表示可以被枚举的，其中会调用到GetEnumerator()。
• IEnumerator表示的是一个枚举器,其实现的是Current,MoveNext,Reset 。

简单的例子

因为Array中本身就实现了GetEnumerator，所以我们可以这么写：

	public class BookInfo {
        public string name {get;set;}
        public int price { get; set; }
        public BookInfo(string name, int price) {
            this.name = name;
            this.price = price;
        }
    }
    
    public class BookIEnumerable : IEnumerable
    {
        private BookInfo[] Books;

        public BookIEnumerable(BookInfo[] books)
        {
            Books = (BookInfo[])books.Clone();
        }
        public BookIEnumerable(List<BookInfo> books)
        {
            Books = books.ToArray();
        }
        public IEnumerator GetEnumerator()
        {
            return Books.GetEnumerator();
        }
    }

			// 我们可以在下面看见我们直接对new出来的对象进行了foreach遍历
			BookInfo[] books =  {
                new BookInfo("蛇皮1",1),
                new BookInfo("蛇皮2",2),
                new BookInfo("蛇皮3",3),
                new BookInfo("蛇皮4",4),
                new BookInfo("蛇皮5",5)
            };
            BookIEnumerable booksIEnumerable = new BookIEnumerable(books);
            foreach (BookInfo book in booksIEnumerable)
            {
                Console.Write("{0} {1}\n", book.name, book.price);
            }

自己实现 IEnumerator

    public class BookIEnumerable : IEnumerable
    {
        private List<BookInfo> Books;

        public BookIEnumerable(BookInfo[] books)
        {
            Books = books.ToList();
        }
        public BookIEnumerable(List<BookInfo> books)
        {
            Books = books;
        }
        public IEnumerator GetEnumerator()
        {
        	//自己实现 IEnumberator
            return new BooksIEnumerator(Books);
        }
    }
    public class BooksIEnumerator : IEnumerator
    {
        private int index = -1;
        private List<BookInfo> Books;

        public BooksIEnumerator(IEnumerable<BookInfo> books) {
            Books = books.ToList();
        }

        public object Current {
            get {
                if (index < 0 || index > Books.Count)
                    return null;
                else
                    return Books[index];
            }
        }

        public bool MoveNext()
        {
            index++;
            if (index < Books.Count && Books[index] != null)
                return true;
            return false;
        }

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

IEnumerable还可以是方法的返回类型

我们使用yield关键字就可以自己做一个Iterator的东西。

    public class Books
    {
        private List<BookInfo> Books;
        public Books(BookInfo[] books)
        {
            Books = books.ToList();
        }
        public Books(List<BookInfo> books)
        {
            Books = books;
        }

        public IEnumerable<BookInfo> getIterator() {
            if (Books.Count > 0)
            {
                foreach (var b in Books) {
                    yield return b;
                }
            }
            else {
                yield return null;
            }
        }
    }

使用起来也很方便

            Books books = new Books(bookList);
            var iterator = books.getIterator();
            foreach (BookInfo book in iterator)
            {
                Console.Write("{0} {1}\n", book.name, book.price);
            }

实用性

举个栗子来说明IEnumerable的实用性

   //书本实体类
   public class BookInfo {
        public string name {get;set;}
        public int price { get; set; }
        public BookInfo(string name, int price) {
            this.name = name;
            this.price = price;
        }
    }
    
    //shopping类
    public class Shopping
    {
        private ValueCalculator vc;

        public Shopping(ValueCalculator vc)
        {
            this.vc = vc;
        }
        public IEnumerable<BookInfo> AllBooks { get; set; }

        public decimal CalculateBookTotal()
        {
            return vc.ValueBooks(AllBooks);
        }
    }

    //计算器
    public class ValueCalculator
    {
        public decimal ValueBooks(IEnumerable<BookInfo> books)
        {
            //IEnumerable中的sum方法
            return books.Sum(b => b.price);
        }

    }