using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        MyType mt = new MyType { OtherProperty = "foo" };

        mt.JsonData.Add("Prop1", "Value1");

        mt.JsonData.Add("Prop2", "Value2");

        string json = JsonConvert.SerializeObject(mt, Formatting.Indented);

        Console.WriteLine(json);

    }

}

​

​

class MyType

{

    public MyType()

    {

        JsonData = new Dictionary<string, object>();

    }

    [JsonExtensionData]

    public Dictionary<string, object> JsonData { get; set; }

    public string OtherProperty { get; set; }

}

​