List<Object> serialization not working with multiple objects - json

Question

I have the following classes
[DataContract]
public class Video
{
[Key]
[DataMember(IsRequired = false)]
public int VideoId { get; set; }
[DataMember(IsRequired = false)]
public int UserId { get; set; }
[Required]
[DataMember ]
public string Title { get; set; }
[Required]
[DataMember]
public virtual IList<Tag> Tags { get; set; }
}
[DataContract]
public class Tag
{
[Key]
[Required]
[DataMember(IsRequired = false)]
public int? TagId { get; set; }
[DataMember(IsRequired = true)]
[Required]
public string Name { get; set; }
[IgnoreDataMember]
public virtual IList<Video> Videos { get; set; }
}
In my WebAPI controller, I call this:
var videos = _service.GetVideos();
return Request.CreateResponse(HttpStatusCode.OK, videos);
Which calls this:
public IList<Video> GetVideos()
{
using (var db = CreateContext())
{
return db.Videos.Include("Tags").ToList();
}
}
Yet over the wire, this is what I get:
[{
"$id": "8",
"tags": [
{
// CORRECT SERIALIZATION
"$id": "9",
"tagId": 1,
"name": "Example",
"count": 5
}
],
"videoId": 18,
"userId": 3,
"title": "Test Video",
"thumbnail": "http://i.imgur.com/gV3J2Uf.png",
"source": "test source"
},
{
"$id": "19",
"tags": [
{
// WTF?
"$ref": "9"
}
],
"videoId": 28,
"userId": 6,
"title": "Test Video",
"thumbnail": "http://i.imgur.com/gV3J2Uf.png",
"source": "test source"
},
{
"$id": "20",
"tags": [
{
// CORRECT AGAIN
"$id": "21",
"tagId": 10,
"name": "funny",
"count": 2
}
],
"videoId": 29,
"userId": 6,
"title": "TEST VID",
"thumbnail": "https://i.imgur.com/SWOQSOf.jpg",
"source": "test source"
},
{
"$id": "22",
"tags": [
{
// INCORRECT
"$ref": "9"
},
{
"$ref": "21"
}
],
"videoId": 30,
"userId": 6,
"title": "TEST VID",
"thumbnail": "https://i.imgur.com/R7lVobX.jpg",
"source": "test source"
}
For some reason - tags is sometimes serializing correctly, and sometimes not. Any idea what I'm doing wrong?

Answer 1

You have circular references in your object graph. They cannot be JSON serialized properly, the serializer detects this condition and automatically makes references ($ref). when you are loading the object graph using EF there are circular references between those objects in memory which cannot be represented correctly in JSON.
I would recommend you breaking the circular references graph by using a view model and then sending the view model over the wire instead of directly returning your autogenerated EF models.