这句linq怎么写成lambda表达式？

　　C# code

　　 var cb = from p1 in b join p2 in bm on p1.BookId equals p2.BookId into bms from p2 in bms.DefaultIfEmpty() select new { p1.BookTitle, BookMessageContent=p2==null "Nothing":p2.BookMessageContent,count=bms.count() };