private void ribbonButton1_Click(object sender, EventArgs e) { foreach (Form f in this.MdiChildren) { if (f is FormSMS) { //MessageBox.Show("Form2 is already opened."); return; } } CloseMdiForm(); _sms = new FormSMS(); _sms.MdiParent = this; _sms.textBox2.Text = port; _sms.Show(); }