In this, my queries are,
Firstly, thank you all for your kind help.. :). Finally i resolved.
While bulk file conversion , my c# process, was getting interrupted , because of some exception occurred in LibreOffice process(soffice). so i started killing my c# process and its child processes and also killed soffice. So for next file, fresh soffice is getting created , as i executed unoconv command.
but still, i get the exception popup, but the c# process , keeps continue.. this code helped me