JsonConverter.cs

public static class MessageConverter
    {
        private static InheritedTypeConverter<IMessage> _default = null;

        public static InheritedTypeConverter<IMessage> Default
        {
            get
            {
                // TODO would be nice to have an attribute I could plug onto classes and have it "auto" find the classes through an assembly
                if (_default == null)
                {
                    _default = new InheritedTypeConverter<IMessage>();
                    _default.AddSelector<EventsMessageImpl>((jsonObj) => { return jsonObj["Events"] != null; });
                    _default.AddSelector<PayloadMessage>((jsonObj) => { return jsonObj["Payload"] != null; });
                }
                return _default;
            }
        }
        public static IMessage DeserializeObject(string json)
        {
            var serializer = new JsonSerializerSettings();
            serializer.Converters.Add(Default);
            return JsonConvert.DeserializeObject<IMessage>(json, serializer);
        }
    }


    // https://skrift.io/articles/archive/bulletproof-interface-deserialization-in-jsonnet/
    public class InheritedTypeConverter<T> : JsonConverter where T : class
    {
        public class TypeSelector
        {
            public Func<JObject, bool> Selector { get; set; }
            public Type Type { get; set; }
        }
        
        private List<TypeSelector> _selectors = new List<TypeSelector>();

        public IReadOnlyList<TypeSelector> Selectors { get { return _selectors.AsReadOnly(); } }

        public override bool CanWrite => false;

        public override bool CanRead => true;

        public override bool CanConvert(Type objectType)
        {
            return objectType == typeof(T);
        }

        public void AddSelector<S>(Func<JObject,bool> selector) where S : T
        {
            _selectors.Add(new TypeSelector() { Selector = selector, Type = typeof(S) });
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            throw new InvalidOperationException("Use default serialization.");
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            var jsonObject = JObject.Load(reader);

            T msg = null;
            foreach(var selector in _selectors)
            {
                // First has priority
                if(msg == null && selector.Selector(jsonObject))
                {
                    msg = (T)Activator.CreateInstance(selector.Type);
                }
            }

            if (msg == null)
                throw new InvalidProgramException("Unable to deserialize string with given type selectors.");

            serializer.Populate(jsonObject.CreateReader(), msg);
            return msg;
        }
    }