private void btnAssign_Click(object sender, EventArgs e) { var studentId = Guid.Parse(((KeyValuePair <string, string>)lstStudent.SelectedItem).Key); var classId = Guid.Parse(((KeyValuePair <string, string>)lstClass.SelectedItem).Key); var res = StudentService.AssignStudent(studentId, classId); MessageBox.Show(res.Message); LoadData(); }