Melina Melina - 3 months ago 48
C# Question

Self referencing loop detected - Getting back data from WebApi to the browser

I am using Entity Framework and having a problem with getting parent and child data to the browser. Here are my classes:

public class Question
{
public int QuestionId { get; set; }
public string Title { get; set; }
public virtual ICollection<Answer> Answers { get; set; }
}

public class Answer
{
public int AnswerId { get; set; }
public string Text { get; set; }
public int QuestionId { get; set; }
public virtual Question Question { get; set; }
}


I am using the following code to return the question and answer data:

public IList<Question> GetQuestions(int subTopicId, int questionStatusId)
{
var questions = _questionsRepository.GetAll()
.Where(a => a.SubTopicId == subTopicId &&
(questionStatusId == 99 ||
a.QuestionStatusId == questionStatusId))
.Include(a => a.Answers)
.ToList();
return questions;
}


On the C# side this seems to work however I notice that the answer objects have references back to the question. When I use the WebAPI to get the data to the browser I get the following message:


The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.

Self referencing loop detected for property 'question' with type 'Models.Core.Question'.


Is this because the Question has Answers and the Answers have a reference back to Question? All the places I have looked suggest having a reference to the parent in the child so I am not sure what to do. Can someone give me some advice on this.

Answer

Is this because the Question has Answers and the Answers have a reference back to Question?

Yes. It cannot be serialized.

EDIT: See Tallmaris's answer and OttO's comment as it is simpler and can be set globally.

GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.Re‌​ferenceLoopHandling = ReferenceLoopHandling.Ignore;

Old Answer:

Project the EF object Question to your own intermediate or DataTransferObject. This Dto can then be serialized successfully.

public class QuestionDto
{
    public QuestionDto()
    {
        this.Answers = new List<Answer>();
    } 
    public int QuestionId { get; set; }
    ...
    ...
    public string Title { get; set; }
    public List<Answer> Answers { get; set; }
}

Something like:

public IList<QuestionDto> GetQuestions(int subTopicId, int questionStatusId)
{
    var questions = _questionsRepository.GetAll()
        .Where(a => a.SubTopicId == subTopicId &&
               (questionStatusId == 99 ||
                a.QuestionStatusId == questionStatusId))
        .Include(a => a.Answers)
        .ToList();

    var dto = questions.Select(x => new QuestionDto { Title = x.Title ... } );

    return dto; 
}