今天在进行EF开发时，遇到一个问题，在进行join查询时，类中的一个集合类型的导航属性，在给它赋值时，将查询出来的结果ToList()后，出错了，linq to entity不支持这种操作，而在linq to sql里这是合法的，在EF中是不行的，所以，使用了替换方法，就是类型强转，代码如下：
var linq = from student in base.GetModel()
join user_classrooms in new TsingDa_NewLearningBarRepository<User_Classroom_R>(base.UnitWork).GetModel()
on student.UserInfoID equals user_classrooms.UserInfoID
join classroom_Info in new TsingDa_NewLearningBarRepository<Classroom_Info>(base.UnitWork).GetModel()
on user_classrooms.ClassroomInfoID equals classroom_Info.ClassroomInfoID
join teacher in base.GetModel()
on classroom_Info.UserInfoID equals teacher.UserInfoID
join classroom_Plan in new TsingDa_NewLearningBarRepository<Classroom_Plan>(base.UnitWork).GetModel()
on user_classrooms.ClassroomInfoID equals classroom_Plan.ClassroomInfoID into list
join classroom_CommonValue_R in new TsingDa_NewLearningBarRepository<Classroom_CommonValue_R>(base.UnitWork).GetModel()
on user_classrooms.ClassroomInfoID equals classroom_CommonValue_R.ClassroomInfoID into valueList
join user_ClassroomPlan_Study in new TsingDa_NewLearningBarRepository<User_ClassroomPlan_Study>(base.UnitWork).GetModel()
.Where(i => i.ClassroomPlanID == 0 && i.StudyStatus == (int)StudyStatus.Study)
on new user_classroom
{
UserInfoID = student.UserInfoID,
ClassroomInfoID = user_classrooms.ClassroomInfoID
} equals new user_classroom
{
UserInfoID = user_ClassroomPlan_Study.UserInfoID,
ClassroomInfoID = user_ClassroomPlan_Study.ClassroomInfoID
} into userplane
let status = userplane.Count() == 0 ? (int)StudyStatus.Study : (int)StudyStatus.Finish
let scale = (list.Count() == 0) ? 0 : userplane.Count() - 1 / list.Count()
select new Classroom_Info_Ext
{
ClassroomInfoID = classroom_Info.ClassroomInfoID,
Title = classroom_Info.Title,
Type = classroom_Info.Type,
EndTime = user_classrooms.EndTime,
Student_Info = student,
Teacher_Info = teacher,
StudyScale = scale,
StudyStatus = (StudyStatus)status,
Classroom_CommonValue_R = (List<Classroom_CommonValue_R>)valueList //这里这前是valueList.ToList()
};
而在分页返回时，也使用了类型转换，这种转换是内存中自动完成，所以性能方面不用考虑
var result = linq.AsQueryable<Classroom_Info>().Where(spec.SatisfiedBy()).OrderBy(i => i.ClassroomInfoID);
return new PagedList<Classroom_Info>(result, pp.PageIndex, pp.PageSize);
对于linq to entity有很多不便的地方，如在使用skip之前，必须要对iqueryable进行ordery操作；在实体赋值时不能使用逻辑判断等等
本文转自博客园张占岭（仓储大叔）的博客，原文链接：EF架构~为导航属性赋值时ToList()的替换方案，如需转载请自行联系原博主。
