void ConnectCOMClick(object sender, EventArgs e) { Button connect = (Button)sender; if (connect.Text == "connect") { // previously disconnected string comPort = string.Empty; try { comPort = comboBoxSelectCOM.SelectedItem.ToString(); } catch { MessageBox.Show(text: "connect SDS011 and select COM port", caption: "COM list error", buttons: MessageBoxButtons.OK); } if (comPort != string.Empty) { bool connectionEstablished = sds011.tryConnectCOM(comPort); if (connectionEstablished) { sds011.startThread(); connect.Text = "disconnect"; connect.BackColor = Color.LightGreen; } } else /* something went wrong, refresh and connect again */ } { }