How to find unique ID/code (can not be changed) for each device ?

If you really want to uniquely identify a computer you have to collect info on all hardware and somehow combine that into a single code...
However this method is extremely problematic from the point of view of the end user... For instance the upgrade of a HDD will break the license (see Microsoft)...

A good method would be providing dongle with a code (like old time cartridge protection)... In that case only the computer with the dongle can have the software run (which does not prevent transfer, but prevents multiple use)...

Of course you can define a set of hardware to define uniqueness - like exclude monitor and HDD - in which case only less frequent changes (like CPU or network card) will brake software... In any case you have to think of a way to re-enable the software with not much effort...

private void GetAllDiskDrives()
    {
        var searcher = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive");

        foreach (ManagementObject wmi_HD in searcher.Get())
        {
            HardDrive hd = new HardDrive();
            hd.Model = wmi_HD["Model"].ToString();
            hd.InterfaceType = wmi_HD["InterfaceType"].ToString();
            hd.Caption = wmi_HD["Caption"].ToString();

            hd.SerialNo =wmi_HD.GetPropertyValue("SerialNumber").ToString();//get the serailNumber of diskdrive

            hdCollection.Add(hd);
        }

 }