using System;

using System.Text.Json;

using System.Collections.Generic;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        var json = @"{""key1"":""value1"",""key2"": {""aa"": ""bb""}}";

        JObject jsonResult = JObject.Parse(json);

        var item = jsonResult["key2"]["aa"].Value<string>();;

        Console.WriteLine($"{item}");

    }

}