call an oracle function from c#

i have a function in oracle with this signature

test_fun(p_nat_no in number,p_number_type in number)  RETURN test_table

in the c# this is the code

stringselect="test_fun";using(OracleConnection connection =newOracleConnection(helper.Global.ConnectionString)){OracleCommand command =newOracleCommand(select, connection);// command.CommandType = CommandType.StoredProcedure;OracleParameterPrmnatNo=newOracleParameter("p_nat_no ", natNo);//natNo
                    command.Parameters.Add(PrmnatNo);OracleParameter pramNumberType =newOracleParameter("p_number_type ", numberType);
                    command.Parameters.Add(pramNumberType);

                    connection.Open();using(OracleDataReader reader = command.ExecuteReader()){if(reader.HasRows){List<test_table>Tickets=newList<test_table>();while(reader.Read()){Tickets.Add(GetPropertyTaxHelper(reader));}returnTickets;}elsereturnnull;}}}

but when trying to call the function from c# an error appears, ? any help? suggestions?