Newtonsoft.Json/Src/Newtonsoft.Json.Tests/Documentation/Samples/Serializer/CustomJsonConverter.cs

using Newtonsoft.Json.Linq;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Newtonsoft.Json.Tests.Documentation.Samples.Serializer
{
  public class CustomJsonConverter
  {
    #region Types
    public class KeysJsonConverter : JsonConverter
    {
      private readonly Type[] _types;

      public KeysJsonConverter(params Type[] types)
      {
        _types = types;
      }

      public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
      {
        JToken t = JToken.FromObject(value);

        if (t.Type != JTokenType.Object)
        {
          t.WriteTo(writer);
        }
        else
        {
          JObject o = (JObject)t;
          IList<string> propertyNames = o.Properties().Select(p => p.Name).ToList();

          o.AddFirst(new JProperty("Keys", new JArray(propertyNames)));

          o.WriteTo(writer);
        }
      }

      public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
      {
        throw new NotImplementedException("Unnecessary because CanRead is false. The type will skip the converter.");
      }

      public override bool CanRead
      {
        get { return false; }
      }

      public override bool CanConvert(Type objectType)
      {
        return _types.Any(t => t == objectType);
      }
    }

    public class Employee
    {
      public string FirstName { get; set; }
      public string LastName { get; set; }
      public IList<string> Roles { get; set; }
    }
    #endregion

    public void Example()
    {
      #region Usage
      Employee employee = new Employee
      {
        FirstName = "James",
        LastName = "Newton-King",
        Roles = new List<string>
        {
          "Admin"
        }
      };

      string json = JsonConvert.SerializeObject(employee, Formatting.Indented, new KeysJsonConverter(typeof(Employee)));

      Console.WriteLine(json);
      // {
      //   "Keys": [
      //     "FirstName",
      //     "LastName",
      //     "Roles"
      //   ],
      //   "FirstName": "James",
      //   "LastName": "Newton-King",
      //   "Roles": [
      //     "Admin"
      //   ]
      // }

      Employee newEmployee = JsonConvert.DeserializeObject<Employee>(json, new KeysJsonConverter(typeof(Employee)));

      Console.WriteLine(newEmployee.FirstName);
      // James
      #endregion
    }
  }
}