Question

如何使用点表示法在linq中执行左外连接？

这是查询表达式：

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 };

这是我试过的：

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

但缺陷显示为IEnumerable<Defect>，而不仅仅是Defect。我也尝试过：

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

这根本就没有编译。所有在线示例似乎都使用（更清晰的）查询语法。

Answer 1

我想你想要这个：

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 });

有关详细信息，请参阅我的Edulinq blog post on query expressions。

Linq - 带点符号的左外连接

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