using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var o = new UserResponse();

        o.Age = "25";

        o.Gender = "Male";

        o.Message = "Hello";

        o.UserInfo = new User();

        o.UserInfo.Id = 1;

        o.UserInfo.FirstName = "John";

        o.UserInfo.LastName = "Doe";

        o.UserInfo.Balance = 1000M;

        var json = JsonConvert.SerializeObject(o, Formatting.Indented, new JsonSerializerSettings {ContractResolver = new CamelCasePropertyNamesContractResolver()}  );

        Console.WriteLine(json);

    }

}

​

public class User

    {

        public int Id { get; set; }

        public string FirstName { get; set; }

        public string LastName{ get; set; }

        public decimal Balance { get; set; }

    }

​

public class UserResponse

    {

        public User UserInfo { get; set; }

        public string Age { get; set; }

        public string Gender { get; set; }

        public string Message { get; set; }

    }