c# - 我的 Enumerable 类不适用于 C# 中的 .where 等 Linq 语句

Question

我希望能够将 Linq 的“.where”语句与实现接口(interface) IEnumerable 的类“Books”(“Book”的列表)一起使用。

//THE PROBLEM IS HERE.
IEnumerable list3 = bookList.Where(n => n.author.Length >= 14);

我得到以下错误:

Error 1 'Assignment2CuttingPhilip.Books' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'Assignment2CuttingPhilip.Books' could be found (are you missing a using directive or an assembly reference?) C:\Users\Alex\Dropbox\cos570 CSharp\Assignment2CuttingPhilip\Assignment2CuttingPhilip\Assignement2PhilipCutting.cs 132 33 Assignment2CuttingPhilip

我的代码如下:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


namespace Assignment2CuttingPhilip
{
    public class Book 
    {
        public string title;
        public string author;

        public Book(string title, string author) {
            this.title = title;
            this.author = author;
        }

        public override string  ToString()
        {  return "Title:\'" + title + "\', Author:" + author;  }

    }

    public class Books : IEnumerable
    {
        private Book[] _books;
        public Books(Book[] bArray){
            _books = new Book[bArray.Length];
            for (int i = 0; i < bArray.Length; i++)
            {_books[i] = bArray[i];}
        }

        IEnumerator IEnumerable.GetEnumerator()
        {return (IEnumerator) GetEnumerator();}

        public BooksEnum GetEnumerator()
        {return new BooksEnum(_books);}

    }

    public class BooksEnum : IEnumerator{
        public Book[] _books;
        int position = -1;

        public BooksEnum(Book[] list){
            _books = list;}

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

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

        object IEnumerator.Current
        { get { return Current; }}

        public Book Current
        {{ try
                { return _books[position];}
                catch (IndexOutOfRangeException)
                {throw new InvalidOperationException();}
        }}
    }

    class Assignement2PhilipCutting
    {
        static void Main(string[] args)
        {
            Book[] bookArray = new Book[3] 
            {
                new Book("Advance C# super stars",  "Prof Suad Alagic"),
                new Book("Finding the right lint",  "Philip Cutting"),
                new Book("Cat in the hat",          "Dr Sues")
            };

            Books bookList = new Books(bookArray);

            IEnumerable List = from Book abook in bookList  
                               where abook.author.Length <= 14
                               select abook;

            IEnumerable list2 = bookArray.Where(n => n.author.Length >= 14);

            //**THE PROBLEM IS HERE**.
            IEnumerable list3 = bookList.Where(n => n.author.Length >= 14);

            foreach (Book abook in List)
            { Console.WriteLine(abook); }
        }
    }
}

这样做应该非常简单，但为什么我不能在 C# 中将我的 Enumerable Books 列表与 Linq 一起使用？难道我不能制作一个可以使用 Fluent Linq 命令查询的可枚举列表吗？

谢谢菲尔

Answer 1

你的 Books类必须实现 IEnumerable<Book> ，不只是 IEnumerable . Where与大多数 LINQ 扩展一样，扩展是为实现 IEnumerable<T> 的对象创建的.此接口(interface)位于 System.Collections.Generic命名空间。

现在您可以使用 Cast() 扩展名:

var list3 = bookList.Cast<Book>().Where(n => n.author.Length >= 14);

这是您可以对仅实现 IEnumerable 的遗留集合执行的操作.但是，在您的场景中，Books类是你的，所以我真的建议你让它实现 IEnumerable<Book> .