c# - linq中的加入子句

Question

我创建了三个表的名称:emps、emp_project、emp_location

现在我想从每个表中选择一列，但是当我执行连接查询时出现这个错误:

The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'.

我正在执行的查询是:

from e in Emp_info
from p in Emp_projects
join l in Emp_locations
    on new { e.User_id , p.Project_id  } equals new { l.User_id, l.Project_id  } into detail
from l in detail
select new
{
    e.Middlename,
    p.Project_name,
    l.Location
}; 
query.Dump("Join query");

不知道是哪个子句导致了错误!

Answer 1

我的猜测是，它试图比较的两个匿名类型不相同(还要确保属性是相同的数据类型)。

改变

on new { e.User_id , p.Project_id  } equals new { l.User_id, l.Project_id  } into detail

到

 on new { UserId = e.User_id, ProjectId = p.Project_id  } equals 
    new { UserId = l.User_id, ProjectId = l.Project_id  } into detail