Set inner object's member as outer object using the object initialization syntax in C#

Question

Context

I have a List<T> of type Question.
Class Question, in turn, contains a List<Answer>.
Class Answer has a member called public Question Question { get; set; } which stores the question of which the answer is for.

I'm using the collection initialization syntax to add a Question item to the list and object initialization to create a new Question. While doing so, I'm also creating new Answer objects using the object initialization syntax(nested).

Problem

How do I set the Question member of the inner Answer class to refer to the enclosing Question object? I know the point at which an Answer is created, the Question is not even fully initialized. But is there any way to obtain the outer Question instance so that I can set it to the inner Answer.

Code

private List<Question> questions = new()
{
    new Question 
    { 
            Id = 1, 
            Text = "Test Question 1", 
            Difficulty = QuestionDifficulty.Easy, 
            Answers = 
            {
                new Answer { Id = 1, Question = [?] },
                new Answer { Id = 2, Question = [?] }   // What should replace [?] here?
            }
    } 
};

Why do you need to do this? It creates a redundant data structure that can lead to bugs in the future. It's better to compute the parent than to store it. — Enigmativity
– Enigmativity, Commented Apr 8, 2021 at 6:44

Sweeper · Accepted Answer · 2021-04-08 06:15:24Z

2

You can't do it in the collection/object initialiser, but I'd argue that you shouldn't do it there in the first place. It's quite error-prone to handwrite what the corresponding question for each answer should be. You might forget to do that sometimes too.

I suggest that you add a custom setter for the Answers property in Question that also sets the answer's Question property:

private List<Answer> answers;
public List<Answer> Answers {
    get => answers;
    set {
        // If you feel like it, you can also set the old answers' questions to null

        answers = value;
        foreach (var answer in answers) {
            answer.Question = this;
        }
    }
}

Then in the object initialiser, initialise the answers' list, rather than just adding to it:

private List<Question> questions = new()
{
    new Question 
    { 
            Id = 1, 
            Text = "Test Question 1", 
            Difficulty = QuestionDifficulty.Easy, 
            Answers = new List<Answer> // <--- here! 
            {
                new Answer { Id = 1 },
                new Answer { Id = 2 }
            }
    } 
};

This compiles to something like:

var question = new Question();
...
var list = new List<Answer>();
var answer1 = new Answer();
answer1.Id = 1;
var answer2 = new Answer();
answer2.Id = 2;
list.Add(answer1);
list.Add(answer2);
question.Answers = list;

As you can see, the setter is called, and the answers' Question property will be set.

answered Apr 8, 2021 at 6:15

Sweeper

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4 Comments

SomeBody Over a year ago

Good approach! Maybe it is even possible to use a custom implementation of IList to set the Question property when a new question is added to the list.

Sweeper Over a year ago

@SomeBody Yeah, if you do that, you don't need new List<Answer> anymore (assuming Answers is initialised in the constructor), but then IMO making a custom list implementation just for this is a bit overkill :-)

Amal K Over a year ago

This is exactly what I needed! Thanks a lot : )

Amal K Over a year ago

@SomeBody That sounds good too. I actually have subclasses for both Question and Answer so I think a custom class AnswerList : IList<Answer> can be reasonable enough to create too. But for now, I'll go with @Sweeper's approach. Thanks :)

SomeBody · Accepted Answer · 2021-04-08 06:09:24Z

2

It is not possible. But you can set the question to all answers after you created them. Do not assign a value to the question property during your use of the collection initialization. In your constructor, you do the following:

foreach(var question in questions)
{
    foreach(var answer in question.Answers)
    {
         answer.Question = question;
    }
}

answered Apr 8, 2021 at 6:09

SomeBody

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