Custom JsonConverter as Attribute

Question

I am trying to figure out how to use a custom JsonConverter as an Attribute. The problem is that I cannot figure out how to get the FOO object within the converter.

Example

[Newtonsoft.Json.JsonConverter(typeof(FOOConverter))]
public interface IFOO
{
    ...
}


public class FOOConverter : Newtonsoft.Json.JsonConverter
{
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jobj = serializer.Deserialize<JObject>(reader);
        ...
        var foo = jobj.ToObject<IFOO>() // Calls the converter again?
    }
}

The .ToObject() will run the converter again and cause a stack overflow, which makes sense since it looks at the attribute, but how can I get the IFOO object then?

Edit: The WriteJson will be simular, but with JObject.FromObject(value);

The usage needs to be flexible, for ex: some properties might be encrypted/encrypted during serialization, other times, there may be property values that needs to be cached. To think of a few use cases.

Answer 1

JSON classes:

[KnownType(typeof(B))]
public class A
{
   public string Name { get; set; }
}

public class B : A
{
   public string LastName { get; set; }
}

Converter code:

    public  class KnownTypeConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return System.Attribute.GetCustomAttributes(objectType).Any(v => v is KnownTypeAttribute);
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            // Load JObject from stream
            JObject jObject = JObject.Load(reader);

            // Create target object based on JObject
            System.Attribute[] attrs = System.Attribute.GetCustomAttributes(objectType);  // Reflection. 

                // Displaying output. 
            foreach (System.Attribute attr in attrs)
            {
                if (attr is KnownTypeAttribute)
                {
                    KnownTypeAttribute k = (KnownTypeAttribute) attr;
                    var props = k.Type.GetProperties();
                    bool found = true;
                    foreach (var f in jObject)
                    {
                        if (!props.Any(z => z.Name == f.Key))
                        {
                            found = false;
                            break;
                        }
                    }

                    if (found)
                    {
                        var target = Activator.CreateInstance(k.Type);
                        serializer.Populate(jObject.CreateReader(),target);
                        return target;
                    }
                }
            }
            throw new ObjectNotFoundException();


            // Populate the object properties

        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }
    }

Usage:

var ret  = JsonConvert.DeserializeObject<A>(json, new KnownTypeConverter());