JSON.Net references become null - serialization

Question

I'm serializing my game state with JSON.Net, but weirdly some references are becoming null. They seem to serialize fine (the $id and $ref seem to be properly set when checking the output json), but the deserialized objectes contain null references where they shouldn't.
My state is as follows: I have a planet, which contains a list of tiles. Each tile knows it neighbours (again a list of tiles).
Here's a little piece of the json, the initial serialization: (most important part is the last line, where neighbours has uses a ref number)
"Tiles": {
"$id": "3",
"$values": [
{
"$id": "4",
"$type": "WarSystems.Tile, Assembly-CSharp",
"_type": 0,
"ID": "161d8ca1-f086-49eb-94ba-0b5b3bbb0921",
"Position": {
"x": 0.0,
"y": 0.0,
"z": -1.71737635
},
"Normal": {
"x": 0.0,
"y": 0.0,
"z": -1.0
},
"Neighbours": [
{
"$id": "5",
"$type": "WarSystems.Tile, Assembly-CSharp",
"_type": 2,
"ID": "f34a2bb1-4a10-49db-b2d5-7980ccc55760",
"Position": {
"x": 0.2789981,
"y": -0.8586796,
"z": -1.460893
},
"Normal": {
"x": 0.1624527,
"y": -0.4999933,
"z": -0.850656152
},
"Neighbours": [
{
"$ref": "4"
},
"Tiles" is the list of tiles in the planet. As you can see, the first tile has a neighbour, and their neighbour has the first tile as his neighbour. Serializing goes fine, all there neighbours are set properly.
However, when I deserialize this, and then reserialize that, I get the following: (now notice that neighbours has a null)
"Tiles": {
"$id": "3",
"$values": [
{
"$id": "4",
"$type": "WarSystems.Tile, Assembly-CSharp",
"_type": 0,
"ID": "161d8ca1-f086-49eb-94ba-0b5b3bbb0921",
"Position": {
"x": 0.0,
"y": 0.0,
"z": -1.71737635
},
"Normal": {
"x": 0.0,
"y": 0.0,
"z": -1.0
},
"Neighbours": [
{
"$id": "5",
"$type": "WarSystems.Tile, Assembly-CSharp",
"_type": 2,
"ID": "f34a2bb1-4a10-49db-b2d5-7980ccc55760",
"Position": {
"x": 0.2789981,
"y": -0.8586796,
"z": -1.460893
},
"Normal": {
"x": 0.1624527,
"y": -0.4999933,
"z": -0.850656152
},
"Neighbours": [
null,
As you can see, the first neighbour is now null. (I also checked the actual deserialized object, it also has the null reference, so it seems the deserializing goes wrong. Of course there is a lot more, but this part shows what's going wrong and the whole json would be a bit much to post.)
Lastly, this is how I (de)serialize:
private static string SerializeState(State state)
{
return JsonConvert.SerializeObject(state, SerializerSettings);
}
private static State DeserializeState(string serialized)
{
return JsonConvert.DeserializeObject<State>(serialized, SerializerSettings);
}
With the SerializerSettings shared between them:
private static JsonSerializerSettings SerializerSettings = new JsonSerializerSettings
{
TypeNameHandling = TypeNameHandling.Objects,
PreserveReferencesHandling = PreserveReferencesHandling.All,
ReferenceLoopHandling = ReferenceLoopHandling.Serialize,
Converters = new List<JsonConverter>() { new Vector3Converter() },
Formatting = Formatting.Indented
};
(Vector3Converter is used to serialize Unity's Vector3. It simply serializes it as an object with 3 field: x, y, and z. As the Position property in the tiles.)
Does anyone know what's going wrong, and how to fix it?
Thanks!
Edit, the Planet and Tile class:
public class Planet
{
public Vector3 Position { get; set; }
public List<Tile> Tiles { get; set; }
public Planet(Vector3 pos, List<Tile> tiles)
{
Position = pos;
Tiles = tiles;
}
}
public class Tile
{
public Guid ID { get; set; }
[JsonRequired]
private TileType _type;
[JsonIgnore]
public TileType Type
{
get { return _type; }
set
{
if (_type != value)
{
_type = value;
if (TypeChanged != null) TypeChanged(_type);
}
}
}
public Vector3 Position { get; set; }
public Vector3 Normal { get; set; }
private List<Tile> _neighbours = new List<Tile>(6); //6 since we use a hex grid
public List<Tile> Neighbours { get { return _neighbours; } set { _neighbours = value; } }
// Events
public event System.Action<TileType> TypeChanged;
public Tile(TileType type, Vector3 pos, Vector3 normal)
{
ID = Guid.NewGuid();
Position = pos;
Normal = normal;
}
public void AddNeighbour(Tile neighbour)
{
_neighbours.Add(neighbour);
}
}

Answer 1

I've solved it. Once I added an empty constructor in my Tile class everything works as intended.
public Tile() { } //for deserialization