模型类的属性不能设置为null

Question

My application is basically just a survey with questions and multiple choice answers. Not all of the questions are intended to be asked of everyone, but only when a person selects a certain answer to a previous question. ie They select C on question 15, then question 15.5 will need to be asked, otherwise question 16 is asked. I am just adding this branching, so everything else has been working.

I added an int property "DependentAnswer" to the "Question" class to hold the ID of an answer which causes the given question to be asked. One answer may cause multiple questions, but the reverse is not true.

When I add to the "Answer" class a property "DependentQuestions" which is a list of int meant to hold the ids of any questions which should be asked if the given answer is chosen, I get this error: The 'DependentAnswer' property on 'Question' could not be set to a 'null' value. You must set this property to a non-null value of type 'System.Int32'.

Here is the important stuff of my question class:

public class Question
{
    [Key]
    public int id { get; set; }

    public string question { get; set; }

    public int DependentAnswer { get; set; }
}

And here is my Answer class. I created a constructor which I meant to populate the DependentQuestions list, but I'm getting an error which seems to have something to do with the LINQ and the fact that DependentAnswer is null in any question which isn't dependent on a, but I'm not experienced enough to know what's wrong.

public class Answer
{
    [Key]
    public int id { get; set; }

    [Required]
    public string answer { get; set; }

    [Required]//which question does this answer belong to
    public int questionId { get; set; }

    public List<int> DependentQuestions { get; set; }

    public Answer()
    {
        using (dbSurvey db = new dbSurvey())
        {
            var _list = db.Questions.Where(q => q.DependentAnswer == id).Select(q => q.id).ToList();
            if (_list.Any())
            {
                DependentQuestions = _list;
            }
            else
            {
                DependentQuestions.Add(0);
            }
        }
    }
}

Answer 1

尝试使DependentAnswer属性为空

public int? DependentAnswer { get; set; }

Answer 2

First initialize your list on your constructor so that it will not trigger an error:

 public Answer()
    {
        DependentQuestions = new List<int>();
        //Your additional logic here
    }

Then if you have null int when using your Question class, then make it nullable:

public int? DependentAnswer { get; set; }