Linq to sql 如何在此linq sql查询中处理空值

如果PrimarySubjectId为null，则上述Linq到SQL查询将失败

PrimarySubjectId在数据库中可以有空值。如果没有subjectId的记录，我希望PrimarySubjectId可以为空。如何处理的空值 主要主体

var query = (from student in dataset.Students
             where student.subjectId == SubjectId || student.subjectId ==dataset.Subjects.FindBySubjecttId(SubjectId).PrimarySubjectId                 
                                 select student)

由于PrimarySubjectId允许null，因此根据linq语句，必须允许属性student.subjectid通过使其可为null的subjectid或int来接受null值？主观性的

    var query = (from student in dataset.Students

                 where student.subjectId == SubjectId || 

 student.subjectId==dataset.Subjects.FindBySubjecttId(SubjectId).PrimarySubjectId                 
select student)