左外LINQ的加入不DefaultIfEmpty

Question

我试图用这个sample：

我要的是唯一的回报：'the书使用id = 2

new Book{BookID=2, BookNm=".NET and COM for Newbies"}, 

我想只有那些没有订单的书。

当我删除DefaultIfEmpty（）它也不起作用。

UPDATE

Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" }; 
      Person terry = new Person { FirstName = "Terry", LastName = "Adams" }; 
      Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" }; 
      Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" }; 

      Pet barley = new Pet { Name = "Barley", Owner = terry }; 
      Pet boots = new Pet { Name = "Boots", Owner = terry }; 
      Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte }; 
      Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry }; 
      Pet daisy = new Pet { Name = "Daisy", Owner = magnus }; 

      // Create two lists. 
      List<Person> people = new List<Person> { magnus, terry, charlotte, arlene }; 
      List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy }; 

      var query = from person in people 
         join pet in pets on person equals pet.Owner into gj      
         where !gj.Any() 
         select person; 

我只是尝试这样的代码，它似乎它的作品！

我退回了人阿琳她没有宠物！

有人可以证实这是一种已知的方法吗？我刚发明它LOL

Answer 1

你误解了左连接的工作原理。 a LEFT JOIN b将返回来自a的所有项目，无论它们是否具有b中的匹配项目。如果我正确理解您的问题，则根本不需要加入，因为您不需要bookOrders的任何其他数据。你可以使用LINQ的All扩展方法：

var query = bookList.Where(b => bookOrders.All(o => o.BookId != b.BookId).ToList(); 

这将返回其没有一本书订单包含该书的ID的所有书籍。我将这些名称放在链接的文章上。