serial port - C#: DataReceived Event handler for serialPort not firing -

i'm writing basic application, 1 of tasks communicating arduino uno card.
write serial communication separate module, forms calls input arduino, , module handle creating , opening of serialport , reading data it.
testing purposes wrote program arduino prints elapsed milliseconds every half second serialport.
populate textbox of form output arduino after press button.
create serialport in serialcomm class, , although attach datareceived event handler it, never seems fire.
here code serialcomm class:

class serialcomm {     private list<string> availableports;     private serialport arduino;     private string receivedtext;      public string[] portlist     {         {             enumports();             return availableports.toarray();         }     }      public string receiveddata     {                 {             return receivedtext;         }     }      public void initialiseserial()     {         arduino = new serialport();         arduino.baudrate = 9600;         arduino.dtrenable = true;         // add event handler         arduino.datareceived += new serialdatareceivedeventhandler(arduino_datareceived);     }      public void enumports()     {         availableports = new list<string>();         foreach (string s in serialport.getportnames())         {             availableports.add(s);         }     }      public void startmc(serialport serialport, string portname)     {         arduino = serialport;         if (arduino.isopen)         {             arduino.close();         }         else         {             //initialise serial port             arduino.portname = portname;                     arduino.open();         }               }      //this never fires==================     private void arduino_datareceived(object sender, serialdatareceivedeventargs e)     {         receivedtext = arduino.readexisting();                }      public void closemc()     {         if (arduino.isopen)         {             arduino.close();             arduino.dispose();         }     } } 

in form call text arduino this: (i have button (button1), combobox selecting com port (combobox1) , textbox on form)

public partial class form1 : form {     serialcomm arduino = new serialcomm();      public form1()     {         initializecomponent();     }      private void form1_load(object sender, eventargs e)     {             //arduino         arduino.initialiseserial();         string[] portlist = arduino.portlist;         foreach (string com in portlist)         {             combobox1.items.add(com);         }          if (portlist.length > 0)         {             combobox1.selecteditem = combobox1.items[0];         }         else         {             combobox1.text = "no microcontroller found!";         }     }      private void button1_click(object sender, eventargs e)     {         arduino.startmc(serialport1, combobox1.selecteditem.tostring());          //as datareceived never fires arduino.receiveddata stays null         if (arduino.receiveddata != null)         {             (int = 0; < 101; i++)             {                 textbox1.appendtext(arduino.receiveddata);             }         }             }      private void form1_formclosing(object sender, formclosingeventargs e)     {         arduino.closemc();     } } 

do see reason why event handler doesn't trigger? thank in advance.

best regards, peter