Linq 左连接 lambda 表达式和结果到一个列表

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; }
}

Trying to write a lambda expression which will do a left join and return a list. The list should contain Books, but BookOrder can be null.

I have tried following which results in build error :

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)

I am not able to change Book or BookOrder classes as this is a 3rd party, ie I have to join on the 3 conditions listed below.

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

Here's your 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();

And here you go with your lambda expression version

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();