using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public class Transaction 

    {

        public string name { get; set; }

        public bool? isMale { get; set; }

        public bool? likesPonies { get; set; }

    }

    public static string Serialize(Transaction transactionData)

    {

        var serialized = JsonConvert.SerializeObject(transactionData, Formatting.None, new JsonSerializerSettings

            {Converters = new List<JsonConverter> { new BoolToStringConverter() },

            });

        return serialized;

    }

    public static void Main()

    {

        var obj = new Transaction()

        {

            name = "Mary",

            isMale = false,

            likesPonies = true

        };

​

        Console.WriteLine(Serialize(obj));

    }

}

​

public class BoolToStringConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(bool);

    }

​

    public override bool CanRead

    {

        get { return false; }

    }

​

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

    {

        throw new NotImplementedException();

    }

​

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

    {

        writer.WriteValue(value.GetType() == typeof(bool?) ? "true" : "false");

    }

}

​