In this article i will show you how to generate invoice number through c# in form of AJ0001/001, AJ0001/002 ... int invoiceNo = 1 ; while ( Console . ReadLine () != "stop" ) { for ( int i = 1 ; i <= 100 ; i ++) { Console . WriteLine ( "{0}/{1}" , invoiceNo . ToString ( "AJ0000" ), i . ToString ( "000" )); } invoiceNo ++; }