ASP.NET MVC C# 列表内列表

Question

I'm using ASP.NET MVC to that enables user to takes exams, each exam contains a list of questions and each question contain multiple answer to form a multiple choice question. Here are my model

 public class Exam
{
    [Key]
    public string ExamID { get; set; }
    public string Name { get; set; }
    public Class Class { get; set; }
    public int Time { get; set; }
    public DateTime Deadline { get; set; }
    public virtual ICollection<Question> Questions { get; set; }
    
    public bool Suffer { get; set; }
}
public class Question
{
    public int ID { get; set; }
    public string question { get; set; }
    public ICollection<String> Answer { get; set; }
    public int CorrectAns { get; set; }
}

It seems fine when I create exam with multiple questions, but when I want to get my list of answers from question, it returns null for my Answer list.Does C# allows to put a list inside a list or it comes from another issue?

This is my code for create and get exam i use in controller: [HttpPost]

    public JsonResult Create(Exam exam,string ClassID)
    {
        if (exam.Time.ToString()!=null  ||exam.Deadline==null)
        {
            exam.Deadline = new DateTime(1990,1,1);
        }       
        Class @class = db.Classes.FirstOrDefault(x => x.ClassID == ClassID);          
        exam.Class = @class;
        exam.ExamID = ClassID +exam.Name;
        db.Exams.Add(exam);
        int a=db.SaveChanges();    
        return Json(a);
    } 
 public ActionResult Details(string id)
    {
        if (id == null)
        {
            return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
        }
        Exam exam = db.Exams.Find(id);
 
        if (exam == null)
        {
            return HttpNotFound();
        }
        foreach(Question question in exam.Questions)
        {
            Debug.WriteLine(question.Answer.Count);
        }

I use Debug.WriteLine to check whether there is any answer string in question and it returns null in this step

Answer 1

It seems that you use Eager loading. So you have to use Include method. In order have multiple levels of depth you have to use ThenInclude after Include method.

db.Exams.Include(x => x.Questions).ThenInclude(y => y.Answers).FirstOrDefault(x => x.ExamID == id);

Official docs:https://docs.microsoft.com/en-us/ef/core/querying/related-data/