Custom JsonConverter self referencing loop

Question

Is there a way I can create my own Custom JsonConverter, which modifies the data before writing out the Json, that works with a nested parent-child structure? Whenever I try at the moment, I end up with a self referencing loop error. I've tried searching for solutions, but I can't quite find anything that matches.

I've created a simple solution to demonstrate the problem.

I have the following model and converter

public class NestedModel
{
    public int Id { get; set; }

    public string Forename { get; set; }

    public string Surname { get; set; }

    public string Custom { get; set; }


    public List<SimpleModel> Children { get; set; }
}

public class NestedModelJsonConverter : JsonConverter
{

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var nestedModel = value as NestedModel;
        nestedModel.Custom = "Modified by Json Converter";

        //This causes a self referencing loop error
        serializer.Serialize(writer, value);

        //This resolves the self referencing loop error, but it does not call my custom Json Converter for any of the Children, and instead uses the default serialization
        //var jo = JObject.FromObject(value);
        //jo.WriteTo(writer);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;

        // Load JObject from stream
        JObject jObject = JObject.Load(reader);

        // Create target object based on JObject
        NestedModel target = new NestedModel();

        // Populate the object properties
        StringWriter writer = new StringWriter();
        serializer.Serialize(writer, jObject);
        using (JsonTextReader newReader = new JsonTextReader(new StringReader(writer.ToString())))
        {
            newReader.Culture = reader.Culture;
            newReader.DateParseHandling = reader.DateParseHandling;
            newReader.DateTimeZoneHandling = reader.DateTimeZoneHandling;
            newReader.FloatParseHandling = reader.FloatParseHandling;
            serializer.Populate(newReader, target);
        }

        return target;
    }

    public override bool CanRead { get { return true; } }

    public override bool CanWrite { get { return true; } }

    public override bool CanConvert(Type objectType) { return typeof(NestedModel).IsAssignableFrom(objectType); }
}

and the test code to use it

string sourceJson = @"{
    ""Id"": 1,
    ""Forename"": ""John"",
    ""Surname"": ""Smith"",
    ""Children"":
    [
        {
            ""Id"": 2,
            ""Forename"": ""Joe"",
            ""Surname"": ""Bloggs"",
            ""Children"": null
        }
    ]
}";

var settings = new JsonSerializerSettings()
{
    Converters = new List<JsonConverter>()
    {
        new NestedModelJsonConverter()
    },
    Formatting = Formatting.Indented
};
var nestedModel = JsonConvert.DeserializeObject<NestedModel>(sourceJson, settings);

string outputJson = JsonConvert.SerializeObject(nestedModel, settings);

However when it tries to Write the Json, it gives a self referencing loop error, presumably when it tries to process the List of Children. I can prevent that error by using a JObject for the conversion, but then that prevents my custom converter from being used for the child elements. I want to have my custom WriteJson method fire for each level of that structure so that I can modify some data before writing it.

Is there a way to do this and get around the self referencing loop error?

Answer 1

After some more work, I found a way to do it, so I thought I'd post it for anyone else having a similar issue. I had to ensure that when Serializing the NestedModel, I individually serialized each property using reflection (then if that property was a list of NestedModel, it called my serializer again for each one).

Here's the model / custom converter

public class NestedModel
{
    public int Id { get; set; }

    public string Forename { get; set; }

    public string Surname { get; set; }

    public string Custom { get; set; }


    public List<NestedModel> Children { get; set; }
}

public class NestedModelJsonConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var nestedModel = value as NestedModel;
        nestedModel.Custom = "Modified by Json Converter";

        JObject jo = new JObject();
        Type type = nestedModel.GetType();

        foreach (PropertyInfo prop in type.GetProperties())
        {
            if (prop.CanRead)
            {
                object propVal = prop.GetValue(nestedModel, null);
                if (propVal != null)
                {
                    jo.Add(prop.Name, JToken.FromObject(propVal, serializer));
                }
            }
        }
        jo.WriteTo(writer);
    }


    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;

        // Load JObject from stream
        JObject jObject = JObject.Load(reader);

        // Create target object based on JObject
        NestedModel target = new NestedModel();


        // Populate the object properties
        StringWriter writer = new StringWriter();
        serializer.Serialize(writer, jObject);
        using (JsonTextReader newReader = new JsonTextReader(new StringReader(writer.ToString())))
        {
            newReader.Culture = reader.Culture;
            newReader.DateParseHandling = reader.DateParseHandling;
            newReader.DateTimeZoneHandling = reader.DateTimeZoneHandling;
            newReader.FloatParseHandling = reader.FloatParseHandling;
            serializer.Populate(newReader, target);
        }

        return target;
    }

    public override bool CanRead { get { return true; } }

    public override bool CanWrite { get { return true; } }

    public override bool CanConvert(Type objectType) 
    { 
        return typeof(NestedModel).IsAssignableFrom(objectType); 
    }
}

and the test code

string sourceJson = @"{
    ""Id"": 1,
    ""Forename"": ""John"",
    ""Surname"": ""Smith"",
    ""Children"":
    [
        {
            ""Id"": 2,
            ""Forename"": ""Joe"",
            ""Surname"": ""Bloggs"",
            ""Children"": null
        }
    ]
}";

var settings = new JsonSerializerSettings()
{
    Converters = new List<JsonConverter>()
    {
        new NestedModelJsonConverter()
    },
    Formatting = Formatting.Indented,
};
var nestedModel = JsonConvert.DeserializeObject<NestedModel>(sourceJson, settings);

string outputJson = JsonConvert.SerializeObject(nestedModel, settings);

the resulting Json created is then

{
  "Id": 1,
  "Forename": "John",
  "Surname": "Smith",
  "Custom": "Modified by Json Converter",
  "Children": [
    {
      "Id": 2,
      "Forename": "Joe",
      "Surname": "Bloggs",
      "Custom": "Modified by Json Converter"
    }
  ]
}