using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using System;

​

namespace ConsoleApp5 {

    class Program {

        static void Main(string[] args) {

            var settings = new JsonSerializerSettings();

            settings.Converters.Add(new SpecialConverter());

            var json = JsonConvert.SerializeObject(new SomeClass { A = 10 }, settings);

            if (!json.Contains("SpecialValue"))

                throw new Exception("Wrong converter was used.");

        }

    }

​

    [JsonConverter(typeof(UsualConverter))]

    class SomeClass {

        public int A;

    }

​

    class UsualConverter : JsonConverter {

        public override bool CanConvert(Type objectType)

            => objectType == typeof(SomeClass);

​

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {

            var jObject = JObject.ReadFrom(reader);

            return new SomeClass {

                A = jObject["A"].ToObject<int>(),

            };

        }

​

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {

            var jObject = new JObject();

            jObject["A"] = ((SomeClass)value).A;

            jObject.WriteTo(writer);

        }

    }

​

    class SpecialConverter : JsonConverter {

        public override bool CanConvert(Type objectType)

            => objectType == typeof(SomeClass);

​

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {

            var jObject = JObject.ReadFrom(reader);

            return new SomeClass {

                A = 100,

            };

        }

​

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {

            var jObject = new JObject();

            jObject["A"] = "SpecialValue";

            jObject.WriteTo(writer);

        }

    }

}

​