c# - 在 LINQ WHERE 子句中有条件地允许 null

Question

工作:

我正在尝试在我的 Contacts 表和我的 Permissions 表之间执行 LEFT OUTER JOIN。我有这个正常工作的基础，无论他们是否有相应的权限，都会得到一个联系人列表。

// Query for Contacts
from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId into permissionGrp
from p in permissionGrp.DefaultIfEmpty()            
where p==null || (p!=null && /* ... condition based on the existence of a permission */)
select new { contact, permission = p };

生成的 WHERE SQL:

WHERE
(t1.PermissionId IS NULL OR 
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... )))

问题:

我想修改以上内容以引入“后备”检查；未按预期工作。

要求:

• 当没有Contact对应的Permission时(即p==null)则只包含基于预定bool 允许的值。

尝试:

我想我可以做 where (p==null && allowed) || ... 像这样:

// Fallback permission
bool allowed = false;

// Query for Contacts
from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId into permissionGrp
from p in permissionGrp.DefaultIfEmpty()

/* Added bool condition 'allowed' to control inclusion when 'p' is null */
where (p==null && allowed) || (p!=null && /* ... condition based on the existence of a permission */)
select new { contact, permission = p };

预期:

当allowed = false时(不接受null权限)

WHERE
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... ))

当allowed = true时(接受null权限)

WHERE
(t1.PermissionId IS NULL OR 
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... )))

实际结果:

即使在 true 时也始终输出为 allowed=false?

WHERE
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... ))

总结:

我希望我只是在做一些很容易修复的傻事。
如何根据给定的 bool 值过滤我的 null 值记录？

Answer 1

您正在执行 GroupJoin这里。所以第一部分的结果，permissionGrp , 是匿名类型 IGrouping<Permission> .这已经是 OUTER JOIN 的等价物。

您可以通过有条件地测试 IGrouping<Permission> 是否满足您的要求。包含元素:

from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId
    into permissionGrp
where allowed || g.Any()
from p in permissionGrp.DefaultIfEmpty()
select new { contact, permission = p };

请注意 from p in permissionGrp再次展平分组，所以 .DefaultIfEmpty()对于 allowed == true 的情况仍然是必要的.