C# 如何使用带有EF 6的Linq扩展方法编写子查询

我是linq的新手，我有3个带有这些列的表

Trainee (ID, TraineeName)
Course (ID, CourseName)
TraineeCourseEnrollment (TraineeID, CourseID, EnrolledDate)

我创建了一个存储过程来使用此查询获取未注册的课程

select * 
from Course 
where ID not in (select CourseID 
                 from TraineeCourseEnrollment 
                 where TraineeID = @traineeid);

---

如何使用扩展方法将相应的linq查询写入此SQL查询？

您必须执行两个查询，第一个是检索要排除的ID，第二个是获取实际课程：

var excludeIDs = db.TraineeCourseEnrollments.Where(w => w.TraineeID == traineeid).Select(s => s.CourseID);
var courses = db.Courses.Where(w =>!excludeIDs.Contains(w.ID)).ToList();

大概是这样的：

扩展方法：

int traineeid = ...;

var query = dc.Courses
              .Where(c => ! dc.TraineeCourseEnrollments
                              .Where(o => o.TrainessID == traineeid)
                              .Select(o => o.CourseID)
                              .Contains(c.ID));

LINQ查询：

int traineeid = ...;

var query =    
    from c in dc.Courses
    where !(from o in dc.TraineeCourseEnrollments
            where o.TraineeID == traineeid
            select o.CourseID)    
           .Contains(c.ID)    
    select c;

Resharper唠叨着让我用

All

而不是

Any

来“简化”这个过程，如图所示：

var notMatchingQueryStyle = from c in prospectus
                            where enrollment.All(r => c.CourseId != r.CourseId)
                            select c;

可能重复的

var notMatchingQueryStyle = from c in prospectus
                            where enrollment.All(r => c.CourseId != r.CourseId)
                            select c;