private void button2_Click(object sender, EventArgs e) { lb_cuorse.Items.Clear(); CoursesDB C = new CoursesDB(); List <string> listOfCourses = new List <string>(); string semester = returnSemestr(cb_semester.SelectedItem.ToString());// SelectedItem.ToString(); string year = returnYear(cb_year.SelectedItem.ToString()); listOfCourses = C.fillCoursesBySenester_list(semester, year); foreach (string Item in listOfCourses) { lb_cuorse.Items.Add(Item); } }