What is the difference between my codes when I use 'dynamic' to use AsQueryable method?

Question

At first,I do not use dynamic,I just use the code like this,and it works well.

  List<Student> result2 = StudentRepository.GetStudent(sex,age).ToList();
  IQueryable rows2 = result2.AsQueryable();

But when I change it to dynamic，it is wrong.

 dynamic result = GetPeopleData(sex,age);
 IQueryable rows = result.AsQueryable();

and I add a method like this,I build the project it show that List do not have the AsQueryable method.How to change it?

 private dynamic GetPeopleData(int sex, int age)
    {
        if(sex>30)
            return StudentRepository.GetStudent(sex,age).ToList();
        else
            return TeacherRepository.GetTeacher(sex, age).ToList();
    }

Answer 1

AsQueryable() is an extension method and those don't work on dynamic.

Depending on what you want to do, there are several possible solutions:

1. Don't use dynamic. Instead, make Student and Teacher implement a common interface (say, IPerson) and use that:

   private IReadOnlyList<IPerson> GetPeopleData(int sex, int age)
   {
       if (sex > 30)
           return StudentRepository.GetStudent(sex, age).ToList();
       else
           return TeacherRepository.GetTeacher(sex, age).ToList();
   }
   
   …
   
   var result = GetPeopleData(sex, age);
   IQueryable<IPerson> rows = result2.AsQueryable();
   

2. Call AsQueryable() as a normal static method:

   dynamic result = GetPeopleData(sex, age);
   IQueryable rows = Queryable.AsQueryable(result);
   

BTW, checking whether sex is over 30 doesn't make any kind of sense to me. You should probably rethink that part of your design.