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.

gambit1 · Accepted Answer · 2017-02-13 11:22:11Z

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());

Thanks for the idea, the problem with Activator.CreateInstance is that within the WriteJson I also need the IFoo object which needs to be converted with JObject.FromObject. FromObject does the same as ToObject

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