How Handle Circular Object References?

By default, the JSON and XML formatters write all objects as values. If two properties refer to the same object, or if the same object appears twice in a collection, the formatter will serialize the object twice. This is a particular problem if your object graph contains cycles because the serializer will throw an exception when it detects a loop in the graph.

Consider the following object models and controller.

public class Employee

{

    public string Name { get; set; }

    public Department Department { get; set; }

}

public class Department

{

    public string Name { get; set; }

    public Employee Manager { get; set; }

}

public class DepartmentsController : ApiController

{

    public Department Get(int id)

    {

        Department sales = new Department() { Name = "Sales" };

        Employee alice = new Employee() { Name = "Alice", Department = sales };

        sales.Manager = alice;

        return sales;

    }

}

Invoking this action will cause the formatter to thrown an exception, which translates to a status code 500 (Internal Server Error) response to the client.

To preserve object references in JSON, add the following code to Application_Start method in the Global.asax file:

var json = GlobalConfiguration.Configuration.Formatters.JsonFormatter;

json.SerializerSettings.PreserveReferencesHandling =

    Newtonsoft.Json.PreserveReferencesHandling.All;

Now the controller action will return JSON that looks like this:

{"$id":"1","Name":"Sales","Manager":{"$id":"2","Name":"Alice","Department":{"$ref":"1"}}}

Notice that the serializer adds an "$id" property to both objects. Also, it detects that the Employee.Department property creates a loop, so it replaces the value with an object reference: {"$ref":"1"}.