problem in opening DB connection in C# to Oracle

I have the following source code (see the end of this msg)  in my  TestOracle app.
But it is always giving error:
System.Data.OracleClient.OracleException: Error while trying to retrieve text for error ORA-12154

   at System.Data.OracleClient.DBObjectPool.GetObject(Object owningObject, Boolean& isInTransaction)
   at System.Data.OracleClient.OracleConnectionPoolManager.GetPooledConnection(String encryptedConnectionString, OracleConnectionString options, OracleConnection owningObject, Boolean& isInTransaction)
   at System.Data.OracleClient.OracleConnection.OpenInternal(OracleConnectionString parsedConnectionString, Object transact)
   at System.Data.OracleClient.OracleConnection.Open()
   at TestOracle.Form1..ctor() in c:\testoracle\testoracle\form1.cs:line 38

What's wrong?

Thanks a lot.

Jennifer

=========================================

String connString="Data Source=Oracle8i;Server=testserver;Uid=jennifer;Pwd=welcome";
                  OracleConnection conn = new OracleConnection(connString);
                  
                  
                  try
                  {
                        conn.Open();
                  }
                  catch(Exception e)
                  {
                        Console.WriteLine(e.ToString());
                  }
                  finally
                  {
                        conn.Close();
                  }