Send raw data to print not working

Question

I want to send raw data to print, avoiding printer selection (fast print).

I am trying to use this helper provided by Microsoft: https://support.microsoft.com/en-us/

Answer 1

Had exactly the same issue with my ZEBRA ZD420 printer. Sending ZPL string to printer only the data light flashing shortly without printing.

I changed only

Marshal.StringToCoTaskMemAnsi(szString);
to 
Marshal.StringToCoTaskMemUTF8(szString);

and it works !