using System;

using System.Collections.Generic;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

​

public class Program

{

    private static readonly JsonSerializerSettings prettyJson = new JsonSerializerSettings()

    {

        ContractResolver = new CamelCasePropertyNamesContractResolver(),

        Formatting = Formatting.Indented

    };

    public static void Main() 

    {   

        var data = new CustomerData()

        {

            CustomerName = "Foo",

            Users = new Dictionary<string, object>() 

            {

                { "1",  new UserData() { UserName = "Fireman", UserPhone = "0118 999 881 999 119 725 ... 3" } },

                { "2",  new UserData() { UserName = "Jenny", UserPhone = "867-5309" } }

            }

        };

        var json = JsonConvert.SerializeObject(data, prettyJson);

        Console.WriteLine(json);

    }

    public class UserData {

        public string UserName;

        public string UserPhone;

    }

​

    public class CustomerData {

        public string CustomerName;

        [JsonExtensionData]

        public Dictionary<string, object> Users;

    }

}