问Join在Devexpress XPO中的LINQ中不起作用

EN

XPQuery<Students> objStudentQuery = new XPQuery<Students>(XpoDefault.Session);
XPQuery<ExamDetails> objExamDetailsQuery = new XPQuery<ExamDetails>(XpoDefault.Session);

var ExamDetails = from exm in objExamDetailsQuery
                          join student in objStudentQuery on exm.StudentId equals student.StudentId
                          select new { student.FirstName, student.LastName, exm.ExamName, exm.Marks, exm.Date };