private void okBtn_Click(object sender, RoutedEventArgs e)
        {
            COURSE   course  = new COURSE();
            TAsLogin taLogin = new TAsLogin();

            try
            {
                if (coursesCombobox.SelectedValue.ToString() == "Structure Programming")
                {
                    course = COURSE.STRUCTURE_PROGRAMMING;
                    taLogin.setCourse(course.ToString());
                }
                else
                {
                    course = COURSE.OOP;
                    taLogin.setCourse(course.ToString());
                }

                taLogin.Show();
                this.Close();
            }

            catch (Exception ex)
            {
                MessageBox.Show("You must choose a course first!");
            }
        }