COM Port drop down list currently lists COM1-COM32 unconditionally. This is a bit clunky, inconvenient and in modern versions of Windows, insufficient. Port numbers can go to 255.
I'd suggest that the list of active COM ports is refreshed on every DropDown event, and refreshed from HKLM\Hardware\DeviceMap\SerialComm key in Windows registry. This key lists all currently active COM ports; it is dynamically refreshed and hence very helpful with hot-pluggable USB-to-COM devices.
