non class use nest json.

example.cs

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

namespace csharp
{
    class Program
    {
        static void Main(string[] args)
        {
            var innerMap = new Dictionary<string, string>();
            innerMap.Add("key", "value");

            var outerMap = new Dictionary<string, Dictionary<string, string> >();
            outerMap.Add("outer_json", innerMap);
            Console.WriteLine(JsonConvert.SerializeObject(outerMap));
        }
    }
}