private void tsAddClass_Click(object sender, EventArgs e) { frmClass frm = new frmClass(); ClassController classController = new ClassController(); Class addClass = new Class(); lap: frm.SetInPut(addClass); frm.ShowDialog(); if (frm.DialogResult == DialogResult.OK) { addClass = frm.getClass(); if (classController.Insert(addClass) == 1) { } else { MessageBox.Show("Lớp đã tồn tại."); goto lap; } showTreeView(); } }