JsonContract.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace JsonTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var product = new Product{
                id="1",
                name="This is the name",
                fields = new Dictionary<string,string> {
                    {"field1","test"},
                    {"field2","test2"}
                }
            };

            var json = JsonConvert.SerializeObject(product);

            Console.WriteLine(json);

            var product2 = JsonConvert.DeserializeObject<Product>(json);

             Console.ReadKey();
        }
    }

    [JsonConverter(typeof(ProductConverter))]
    public class Product
    {
        public Product()
        {
            fields = new Dictionary<string, string>();
        }

        public string id { get; set; }
        public string name { get; set; }
        public Dictionary<string, string> fields { get; set; }
    }

    public class ProductConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return objectType == typeof(Product);
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            var product = value as Product;

            writer.WriteStartObject();
            writer.WritePropertyName("id");
            writer.WriteValue(product.id);
            writer.WritePropertyName("name");
            writer.WriteValue(product.name);
           
            foreach (var item in product.fields)
            {
                writer.WritePropertyName(item.Key);
                writer.WriteValue(item.Value);
            }
            writer.WriteEndObject();
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            var product = existingValue as Product ?? new Product();

            while (reader.Read())
            {
                if (reader.TokenType == JsonToken.EndObject) continue;

                var value = reader.Value.ToString();
                switch (value)
                {
                    case "id":
                        product.id = reader.ReadAsString();
                        break;
                    case "name":
                        product.name = reader.ReadAsString();
                        break;
                    default:
                        product.fields.Add(value, reader.ReadAsString());
                        break;
                }

            }

            return product;
        }
    }
}