how to get all serail port com in silverlight application

//  com = AutomationFactory.CreateObject("ComInterOp_SeeLevel828_Utility");
com = AutomationFactory.GetObject("InteropComObjects.IO.Ports.SerialPort");
 dynamic count = com.GetDeviceCount();
 StringBuilder sb = new StringBuilder();

 List<dynamic> devices = new List<dynamic>();
 for (int i = 1; i <= 9; i++)
 {
     devices.Add("COM" + i);
 }

 for (int i = 0; i < count; i++)
 {
     devices.Add(com.GetDevice(i));

 }
 cmdSerial.ItemsSource = devices;

com = AutomationFactory.GetObject("InteropComObjects.IO.Ports.SerialPort");