Linq 左连接 lambda 表达式和结果到列表

Question

public class JoinModel
{
        public Book Book { get; set; }
        public BookOrder BookOrder { get; set; }
}

public class Book
{
        public int BookID { get; set; }
        public string  UniqueID{ get; set; }
        public int Year { get; set; }
        public int BookNumber { get; set; } 
        public int Value { get; set; }

}

public class BookOrder
{
        public int BookOrderID { get; set; }
        public string  UniqueID{ get; set; }
        public int Year { get; set; }
        public int BookNumber { get; set; }
        public DateTime OrderDate { get; set; }
}

正在尝试编写一个 lambda 表达式，它将执行左连接和 return 列表。该列表应包含书籍，但 BookOrder 可以为空。

我试过以下导致构建错误：

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<...BookOrder> to ..BookOrder An explicit conversion exists (are you missing a cast?) on Line 5 (red squigles on bko)

我无法更改图书或图书订单类，因为这是第 3 方，即我必须满足下面列出的 3 个条件才能加入。

List<JoinModel> lstJoinModel = new List<JoinModel>();

Line 1 - lstJoinModel  = Context.Books
Line 2 - .GroupJoin(Context.BookOrder,
Line 3 - bk => new {     bk.UniqueID, bk.Year, bk.PostingId },
Line 4 - bko => new {     bko.UniqueID, bko.Year, bko.BookNumber },
Line 5 - (bk, bko) => new     JoinModel { Book = bk, BookOrder = bko })
Line 6 - .Where(r => r.Book.Value >     0).ToList();

Answer 1

这是您的 linq：

List<JoinModel> lstJoinModel = (from bk in Context.Books
                                join bko in Context.BookOrder on new { bk.UniqueID, bk.Year, bk.BookNumber } equals new { bko.UniqueID, bko.Year, bko.BookNumber }
                                into bd
                                from bd2 in bd.DefaultIfEmpty()
                                where bk.Value > 0
                                select new JoinModel { Book = bk, BookOrder = bd2 }  
                               ).ToList();

这里是你的 lambda 表达式版本

List<JoinModel> lstJoinModel = Context.Books.GroupJoin(Context.BookOrder,
                               bk => new { bk.UniqueID, bk.Year, bk.BookNumber },
                               bko => new { bko.UniqueID, bko.Year, bko.BookNumber },
                               (x, y) => new { Book = x, BookOrder = y })
                               .SelectMany(x => x.BookOrder.DefaultIfEmpty(),
                               (x, y) => new JoinModel
                               {
                                   Book = x.Book,
                                   BookOrder = y
                               })
                              .Where(r => r.Book.Value > 0).ToList();

Linq 左连接 lambda 表达式和结果到列表

Linq left join lambda expression and result to a list

c#

linq

lambda

left-join