Linq - 带点符号的左外连接

Question

如何使用点符号在linq中执行左外连接？

这里的查询表达式：

var query = from u in db.Users 
      join d in db.Defects on u.userID equals d.userID into defectsGroup 
      from d in defectsGroup.DefaultIfEmpty() 
      select new { u, d }; 

这里是我的尝试：

var query2 = db.Users.GroupJoin(db.Defects.DefaultIfEmpty(), 
       u => u.userID, 
       d => d.userID, 
       (user, defect) => new { user, defect }); 

但缺陷是显示为IEnumerable<Defect>而不仅仅是Defect。我也试过：

var query2 = db.Users.GroupJoin(db.Defects, 
       u => u.userID, 
       d => d.userID, 
       (user, defect) => new { user, defect.DefaultIfEmpty() }); 

哪个根本不会编译。所有在线示例似乎都使用（更清晰）查询语法。

Answer 1

我想你想要这样的：

var query2 = db.Users.GroupJoin(db.Defects, 
           u => u.userId, 
           d => d.userID, 
           (u, defectsGroup) => new { u, defectsGroup}) 
        .SelectMany(z => z.defectsGroup.DefaultIfEmpty(), 
           (z, d) => new { z.u, d }); 

见我Edulinq blog post on query expressions了解更多详情。