using System;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        var str = "[{\"CAT\":\"OK\",\"MSG\":\"SESION-OK\",\"EXTRA\":{\"ID\":\"3\",\"NOM\":\"CHARLS\"}}]";

        var j = JArray.Parse(str);

        var token = j[0];

        var obj = token.ToObject<dynamic>();

        Console.WriteLine(obj.CAT);

        Console.WriteLine(obj.MSG);

        Console.WriteLine(obj.EXTRA);

        Console.WriteLine(obj.EXTRA.ID);

        Console.WriteLine(obj.EXTRA.NOM);

    }

}