using System;

public class Program

{

    private  static string _sessionId = "";

    public static void Main()

    {

        start(null);

        start("Not null continueSession");

        _sessionId = "Not null session id";

        start(null);

        start("Not null continueSession");

    }

    public static void start(string continueSession = null)

    {

        if (session_id() == "" && string.IsNullOrEmpty(continueSession)) {

            Console.WriteLine("New Session");

        }

        else{

            Console.WriteLine($"Continue Session with {continueSession ?? "NULL"}");

        }

    }

    public static string session_id()

    {

        return _sessionId;

    }

​

}