using System;

using System.Collections.Generic;

using System.Text.Json;

​

public class Program

{

    public static void Main()

    {

​

        var authCookie = new Dictionary<string, string>();

        authCookie.Add("Nickname", "abc");

        var cookieSerialized = JsonSerializer.Serialize(authCookie);

        Console.WriteLine(cookieSerialized);

        var pluginCookie = JsonSerializer.Deserialize<Dictionary<string,object>>(cookieSerialized);

        Console.WriteLine(pluginCookie["Nickname"]);    

    }

}