强制转换为值类型'System.Int32'失败，因为具体化值为 null

Question

我有一个Linq查询，但它给出了上面的错误，我想是由于NULL，因为我有Module、Block和termont作为可为NULL的int，当termont列的值大于Module和Block时，它将为NULL，当Module和Block为NULL时，它的值大于semster的值如何处理下面的查询。

var test1 = (from c in db.StudentCoursesAssigned 
join e in db.Years 
on c.Year_Id equals e.Id into table1 from e in table1.DefaultIfEmpty()
join cc in db.Courses
on c.Course_Id equals cc.Course_Id  into table2 from cc in table2.DefaultIfEmpty()
join g in db.grades 
on c.Grade equals g.Id into table3 from g in table3.DefaultIfEmpty()
join m in db.Moduels
on c.Module_Id equals m.Id into table4 from m in table4.DefaultIfEmpty()
join p in db.Programs
on c.Program_Id equals p.Id into table5 from p in table5.DefaultIfEmpty()
join b in db.Blocks
on c.Block_Id equals b.Id  into table6 from b in table6.DefaultIfEmpty()
join s in db.Semesters
on c.Semster_Id equals s.Semester_Id into table7 from s in table7.DefaultIfEmpty()
join ss in db.Students
on c.Student_id equals ss.Student_Id into table8 from ss in table8.DefaultIfEmpty()
select new
{
Student_Name=ss.Student_FName,
Course=cc.Course_Name,
Active=c.Active,
Course_start_date=c.Enrolment_Start,
Course_End_date=c.Enrolment_End,
Grade=g.Name,
Module=m.Name,
Program=p.Program_Title,
Year=e.Id,
Semester=s.Semester_Title,
blocks=b.Id


}).ToList();

Answer 1

这是解决的正确Linq查询

var test1 = (from c in db.StudentCoursesAssigned
join e in db.Years
on c.Year_Id equals e.Id into table1 from e in table1.DefaultIfEmpty()
join cc in db.Courses
on c.Course_Id equals cc.Course_Id into table2 from cc in table2.DefaultIfEmpty()
join g in db.grades
on c.Grade equals g.Id into table3 from g in table3.DefaultIfEmpty()
join m in db.Moduels
on c.Module_Id equals m.Id into table4 from m in table4.DefaultIfEmpty()
join p in db.Programs
on c.Program_Id equals p.Id into table5 from p in table5.DefaultIfEmpty()
join b in db.Blocks
on c.Block_Id equals b.Id into table6 from b in table6.DefaultIfEmpty()
join s in db.Semesters
on c.Semster_Id equals s.Semester_Id into table7 from s in table7.DefaultIfEmpty()
join ss in db.Students
on c.Student_id equals ss.Student_Id into table8 from ss in table8.DefaultIfEmpty()
select new
{
Student_Name = ss.Student_FName == null ? "No Value" : ss.Student_FName,
Course = cc.Course_Name == null ? "No Value" : cc.Course_Name,
Active = c.Active ,
Course_start_date = c.Enrolment_Start,
Course_End_date = c.Enrolment_End ,
Grade = g.Name == null ? "No Value" : g.Name,
Module = m.Name == null ? "No value" : m.Name,
Program=p.Program_Title == null ? "No Value" : p.Program_Title,
Year=e.Name,
Semester=s.Semester_Title == null ? "No Value" : s.Semester_Title,
blocks=b.Name == null ? "No Value":b.Name,
student_Id=ss.Student_Id,
id=c.Id,

}).ToList();
```