第四章开始了 LinQ to Object 的讲解（Entity 用了很多linq的语法，如果Linq 掌握好了Entity很好学了）：

第四第五章 需要多加练习例子，熟练linq 的语法。

第四章的例子做完了，主要讲了一些常用的Linq 语法。

1.select select many

2. orderby 

3. group, join group, left join, cross join, inner join, nested query

4.where .......

 5. take() skip()

写了几个小例子：

 var publishersource = from bookbb in books

                                   select new

                                   {

                                       Name = from pub in Publishers

                                              where pub.Name == bookbb.Publisher.Name

                                              select pub,

                                       bookbb.Title,

                                       bookbb.Price

                                   };

             var groupsource = from groupbook in books

                               group groupbook by groupbook.Publisher into PublisherGroupName

                               select new { Name = PublisherGroupName.Key.Name, Books = PublisherGroupName };

             var groupjoinsource = from publishersor in Publishers

                              join booksor in books

                              on publishersor equals booksor.Publisher into PublisherGroup

                              select new { Name = publishersor.Name, Books = PublisherGroup };

             var leftjoingroup = from publisher in Publishers

                                   join booksor in books

                                   on publisher equals booksor.Publisher into GroupPublisher

                                   from book in GroupPublisher.DefaultIfEmpty()

                                   select new { Publisher = publisher.Name,

                                                Book = book == default(Book)?"no books":book.Title };

             var crossingJoin = from publisher in Publishers

                                from book in books

                                select new

                                {

                                    Name = publisher.Name,

                                    Title = book.Title

                                };

第五章主要讲性能：

1. nongeneric collection need to : Cast, ofType,  explicitly convert to object.

2.分组使用多个元素  group by more than one characters.