void InitializePathDirectory(string dir)
    {
        if (!Directory.Exists(dir))
        {
            Directory.CreateDirectory(dir);
        }
    }

    void InitializeFileDirectory(string file)
    {
        if (!File.Exists(file))
        {
            File.Create(file);
        }
    }

private string dirname = $"{path}\Logs";
    private string filename = $"{path}\Logs\{DateTime.Now.ToString("dd.MM.yyyy")}.csv";

public void onExit()
    {
       //var a is the DateTime, when the application starts
       var b = DateTime.Now;

            string temp = Convert.ToString(b - a);
            string[] time = temp.Split('.');

            File.AppendAllText(filename, $"Time:; {time[0]}n");
            File.AppendAllText(filename, $"Starttime:; {a.ToString("HH:mm:ss")}n");
            File.AppendAllText(filename, $"Endtime:; {b.ToString("HH:mm:ss")}nn");
    }