linq：OrderByDescending 按指定表达式对集合倒序排序；延迟

第一步：查询出学生成绩表中的数据

using (var db = new SMSPModel.SMSPEntities())
            {
                db.CommandTimeout = 0;
                var tb = (from p in db.tbExamMark
                          where p.tbExamCourse.ID == this.DrplstExamCourse1.ExamCourseId
                          && p.RecordStatus == decimal.One
                          && p.tbExamCourse.RecordStatus == decimal.One
                          && p.tbExamCourse.tbExamBasis.ID == this.DrplstExam1.ExamId
                          && p.tbExamCourse.tbCourseBasis.RecordStatus == decimal.One
                          && p.tbExamCourse.tbCourseBasis.tbSubjectBasis.RecordStatus == decimal.One
                          && p.tbExamCourse.tbCourseBasis.tbSubjectBasis.tbAreaBasis.RecordStatus == decimal.One
                          && p.tbStudentBasis.RecordStatus == decimal.One
                          && p.tbStudentBasis.StudentStatus == decimal.One
                          orderby p.tbStudentBasis.StudentCode
                          select new ExamMark
                                 {
                                     StudentId = p.tbStudentBasis.ID,
                                     StudentCode = p.tbStudentBasis.StudentCode,
                                     StudentName = p.tbStudentBasis.StudentName,
                                     ClassName = (from p0 in db.tbClassStudent
                                                  where p0.RecordStatus == decimal.One &&
                                                        p0.tbStudentBasis.ID == p.tbStudentBasis.ID &&
                                                        p0.tbClassBasis.tbYear.FromDate <= p.tbExamCourse.tbExamBasis.tbYear.FromDate &&
                                                        p0.tbClassBasis.tbYear.ToDate >= p.tbExamCourse.tbExamBasis.tbYear.ToDate &&
                                                        p0.tbClassBasis.tbYear.tbYearGroup.GroupNo == decimal.One
                                                  select p0.tbClassBasis.ClassName).FirstOrDefault(),
                                     AppraiseMark = p.AppraiseMark == decimal.MinusOne ? null : (decimal?)p.AppraiseMark,
                                     Mark1 = p.Mark1 == decimal.MinusOne ? null : (decimal?)p.Mark1,
                                     Mark2 = p.Mark2 == decimal.MinusOne ? null : (decimal?)p.Mark2,
                                     TotalMark = p.TotalMark == decimal.MinusOne ? null : (decimal?)p.TotalMark,
                                     TotalGradeRank = p.TotalGradeRank == decimal.MinusOne ? null : (decimal?)p.TotalGradeRank,
                                     SegmentMark = p.SegmentMark == decimal.MinusOne ? null : (decimal?)p.SegmentMark,
                                     SegmentGradeRank = p.SegmentGradeRank == decimal.MinusOne ? null : (decimal?)p.SegmentGradeRank,
                                     LevelName = p.tbExamLevel.LevelName,
                                     GetPoint = p.tbExamLevel.LevelValue > decimal.Zero ? p.tbExamCourse.tbCourseBasis.CoursePoint : decimal.Zero,
                                     StudyPoint = p.tbExamCourse.tbCourseBasis.CoursePoint * (p.tbExamLevel.LevelName == "---" ? decimal.Zero : p.tbExamLevel.LevelValue)
                                 }).Distinct().OrderByDescending(c => c.SegmentMark).ToList();

第二步，根据SegmentMark  综合分 进行排名

 //重新排名(综合分排名)
                var studentScoresRankList = (from p in tb
                                             select new ExamMark
                                             {
                                                 StudentId = p.StudentId,
                                                 StudentCode = p.StudentCode,
                                                 StudentName = p.StudentName,
                                                 ClassName = p.ClassName,
                                                 AppraiseMark = p.AppraiseMark,
                                                 Mark1 = p.Mark1,
                                                 Mark2 = p.Mark2,
                                                 TotalMark = p.TotalMark,
                                                 TotalGradeRank = p.TotalGradeRank,
                                                 SegmentMark = p.SegmentMark,
                                                 LevelName = p.LevelName,
                                                 GetPoint = p.GetPoint,
                                                 StudyPoint = p.StudyPoint,
                                                 SegmentGradeRank = tb.Select(c => c.SegmentMark).ToList().IndexOf(p.SegmentMark) + 1,
                                             }).OrderBy(c => c.TotalGradeRank).ToList();


                return studentScoresRankList;

　　然后排名就出来了，然后重新绑定就可以了。