To anyone still looking for an answer:

JSONParser parser = new JSONParser();
JSONObject json = (JSONObject) parser.parse(stringToParse);

You can use:

JSONObject jsonObject = new JSONObject();
jsonObject.toString();

And if you want to get a specific value, you can use:

jsonObject.getString("msg");

or Integer value

jsonObject.getInt("codeNum");

For long Integers

jsonObject.getLong("longNum");

When there are internal properties mark them as a JsonProperty, otherwise go with SimpleSamples recommendation.

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

namespace ObjectToJson
{
    class Program
    {
        static void Main(string[] args)
        {
            var i1 = new Item
            {
                Name = "test1",
                Value = "result1"
            };

            var i2 = new Item
            {
                Name = "test2",
                Value = "result2"
            };

            ReturnData returnData = new ReturnData();

            var items = new List {i1, i2};


            returnData.Type = "type1";
            returnData.Items = items;

            var json = JsonConvert.SerializeObject(returnData, Formatting.Indented);

            Debug.WriteLine(json);

        }
    }
    class ReturnData
    {
        [JsonProperty]
        internal string Type { get; set; }
        [JsonProperty]
        internal List Items { get; set; }
    }

    public class Item
    {
        public string Name { get; set; }
        public string Value { get; set; }
    }
}

Output

{
  "Type": "type1",
  "Items": [
    {
      "Name": "test1",
      "Value": "result1"
    },
    {
      "Name": "test2",
      "Value": "result2"
    }
  ]
}