If your are using Serial Port class then you can use the below method to get the comports available
using System.IO.Ports;
SerialPort sp=new SerialPort(string portName,int baudRate,Parity parity,int dataBits,StopBits stopBits);
sp.Open();
string[] arr=Serialport.GetPortNames();
foreach(var item in arr)
{
Combobox1.Items.Add(item);
}
sp.Close();
In Serial port class,GetPortNames() will fetch all the com ports of PC
GetPortNames -Gets an array of serial port names for the current computer.
Hope this will help you.