private void Button1_Click(object sender, EventArgs e) { if (listBox1.SelectedItem != null && listBox2.SelectedItem != null) { usersRepository.AttachStudent(listBox1.SelectedItem.ToString(), gradesRepository.GetGroupId(listBox2.SelectedItem.ToString())); listBox1.Items.Clear(); List <Student> students = usersRepository.GetStudents(0); foreach (Student student in students) { listBox1.Items.Add(student.Name + " " + student.Surname); } listBox2.Items.Clear(); List <Group> groups = gradesRepository.GetGroups(); foreach (Group group in groups) { listBox2.Items.Add(group.Name); } } }