.net+oracle问题

System.Data.OracleClient.OracleException: ORA-12154: TNS: 无法解析指定的连接标识符！Connection = new OracleConnection("Data Source=OnlineExam;User ID=system;Password=zero;");
行 39:                 Connection.Open();//这一行显示出错
行 40:             }
行 41:             else
 
 public class DataBase
    {
        //私有变量，数据库连接
        protected OracleConnection Connection;
        protected string ConnectionString;        //构造函数
        public DataBase()
        {
            string mysqlstr = "Data Source=OnlineExam;Persist Security Info=True;User ID=system;Password=zero;Unicode=True";
            OracleConnection mycnn = new OracleConnection(mysqlstr);
            ConnectionString = mysqlstr;            //ConnectionString = "Data Source=PC-201109021256;DataBase=test;User ID=sa;Password=123456;";
        }
        //保护方法，打开数据库连接
        private void Open()
        {
            //判断数据库连接是否存在
            if (Connection == null)
            {
                //不存在，新建并打开                Connection = new OracleConnection("Data Source=OnlineExam;User ID=system;Password=zero;");
                Connection.Open();
            }
            else
            {
                //存在，判断是否处于关闭状态
                if (Connection.State.Equals(ConnectionState.Closed))
                    Connection.Open();   //连接处于关闭状态，重新打开
            }
        }