how to recieve data via com port?

In order to access comport you need to have namespace
using System.IO.Ports;
then in button click event write this
{
then need to create an object for serial port like this

SerialPort obj = new SerialPort("COM1");//But be shore that the bardrate matches between hardware and computer system
            obj.Open();
string a = obj.ReadExisting();
            obj.Close();
MessageBox.Show(a);


}

Hope this may help you