private void ribbonButton3_Click(object sender, EventArgs e) { foreach (Form f in this.MdiChildren) { if (f is AddSiswa) { //MessageBox.Show("Form2 is already opened."); return; } } CloseMdiForm(); _addSiswa = new AddSiswa(); _addSiswa.MdiParent = this; _addSiswa.groupBox1.Text = "Tambah Siswa"; _addSiswa.main = this; _addSiswa.Show(); }