(WebApi) Can't serialize collection of objects to Json

Question

In my WebApi controller I have a few methods that return objects retrieved from a database which are serialized to Json. Everything works fine if a method serializes and returns only a single object, it fails when it tries to serialize a collection of objects.

This is my model class:

    [Table("Athlete")]
    public partial class Athlete
    {
        public Athlete()
        {
            Event = new HashSet<Event>();
            User = new HashSet<User>();
        }

        [Required]
        [StringLength(32)]
        [DisplayName("First name")]
        public string FirstName { get; set; }

        [Required]
        [StringLength(32)]
        [DisplayName("Last name")]
        public string LastName { get; set; }

        [StringLength(32)]
        [DisplayName("Sport")]
        public string Sport { get; set; }

        [Key]
        [Column(TypeName = "numeric")]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public decimal Athlete_ID { get; set; }

        [DataMember]
        [Column(TypeName = "numeric")]
        public decimal? Team_Team_ID { get; set; }

        [NotMapped]
        [DisplayName("Team")]
        public string TeamName { get; set; }

        [JsonIgnore]
        public virtual Team Team { get; set; }

        [JsonIgnore]
        public virtual ICollection<Event> Event { get; set; }

        [JsonIgnore]
        public virtual ICollection<User> User { get; set; }
    }

This works fine:

[HttpGet]
public IHttpActionResult GetById(int id)
{
    var athlete =  _db.Athlete
        .Where(a => a.Athlete_ID == id)
        .FirstOrDefault();

    if (athlete != null)
    {
        return Json<Athlete>(athlete);
    }

    return NotFound();
}

The following method causes a serialization error (System.InvalidOperationException) (The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.)

The inner exception's message is "Self referencing loop detected for property 'ApplicationInstance' with type 'ASP.global_asax'. Path 'Request.Properties.MS_HttpContext.ApplicationInstance.Context'."

[HttpGet]
public IHttpActionResult GetAllAthletes()
{
    var athletes = _db.Athlete.ToArray();

    if (athletes != null)
    {            
        return Ok(Json<IEnumerable<Athlete>>(athletes));
    }

    return NotFound();
}

I've already tried to change the serialization settings in WebApiConfig.cs like in this question but nothing has worked so far.

Any help would be appreciated.

Have you tried to assign the result of the `Json<>()` call to a variable and pass that to `Ok`? — Bernhard Döbler, Aug 16 '15 at 11:15
I finally tried it myself - in Visual Basic in Visual Studio 2015 - and got the same error. I removed the `OK` wrapped around the `Json` and got a result. The server returned a JSON object and HTTP Status 200. Why would you want to wrap it in OK yourself? — Bernhard Döbler, Aug 22 '15 at 19:27
Thanks for the tip. In this simplified scenario it would indeed be sufficient to simply wrap an Enumerable<> around the Json. However, with this approach you cannot return any other status than 200. My class could be slightly tweaked so it can return other statuses as well, though. — rafaLiusz, Aug 25 '15 at 13:05

score 0 · Accepted Answer · answered Aug 16 '15 at 15:31

I've managed to find a way to work-around this in a semi-elegant manner. I'm not completely happy with this but a man's gotta do what a man's gotta do.

In case anyone needs this in the future:

Create a class that implements the IHttpActionResult interface:

public class MyJsonResult : IHttpActionResult
    {
        object _value;
        HttpRequestMessage _request;

        public MyJsonResult(object value, HttpRequestMessage request)
        {
            _value = value;
            _request = request;
        }

        public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
        {
            var response = new HttpResponseMessage()
            {
                Content = new StringContent(JsonConvert.SerializeObject(_value), Encoding.UTF8, "application/json"),
                RequestMessage = _request,
                StatusCode = HttpStatusCode.OK
            };

            return Task.FromResult(response);
        }
    }

Then use it in a following way:

[HttpGet]
public IHttpActionResult GetAllAthletes()
{
    var athletes = _db.Athlete;

    if (athletes != null)
    {
        return new MyJsonResult(athletes, Request);
    }

    return NotFound();
}

(WebApi) Can't serialize collection of objects to Json

1 Answers1