hi

if you are using oracle instant client , you dont have oracle connection configuration files available.
you have to use the entire tnsnames block on your connection string.

try

using system.data.oracleclient;
 
oracleconnection a = new oracleconnection();
 
a.connectionstring = "User ID=" + /*your_oracle_user*/ +
                                        ";Password=" + /*your_oracle_password*/ +
                                        ";Data Source=" +
                                        "(DESCRIPTION = " +
                                        " (ADDRESS = (PROTOCOL = TCP)(HOST = " + /*Your_host_name_or_ipaddress*/ + ")(PORT = 1521)) " +
                                        " (CONNECT_DATA = " +
                                        " (SERVER = DEDICATED) " +
                                        " (SID = " + /*Your_Sid_NAME*/ + ") " +
                                        " ) " +
                                        ")";
a.open();