Connection string that my app is using to connect to DB is the following:
    private const string oradb = "Data Source=(DESCRIPTION=(ADDRESS_LIST="
                    + "(ADDRESS=(PROTOCOL=TCP)(HOST=host.name)(PORT=1521)))"
                    + "(CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME=service.name)));"
                    + "User Id=myusername;Password=mypass;";
In all DB access points of my app I am using the following pattern:
        OracleConnection conn = new OracleConnection(oradb);
        try
        {
            Console.WriteLine("Opening DB Connection...");
            conn.Open();
            string queryString = string.Format(@"SELECT ...");
            using (OracleCommand command = new OracleCommand(queryString, conn))
            {
                using (OracleDataReader reader = command.ExecuteReader())
                {
                    while (reader.Read())
                    {
                     ...
                    }
                }
            }
        }
        catch (Exception e)
        {
            Console.WriteLine("Exception occured during DB access: {0}", e.Message);
            dbr.Error = e.Message;
        }
        finally
        {
            Console.WriteLine("Closing DB connection");
            conn.Close();
            conn.Dispose();
        }
For sure I am properly handling exceptions and in try/catch/finally closing AND disposing connection object. However, often I am receiving oracle service message that I am holding oracle sessions. Moreover, if I just leave my app open and next day try to make operation, I am getting ora-12537 network session end of file exception first time, then second attempt is going through. After some reading it looks like I have to disable connection pool. If this is the right way to solve, how to disable pool? If not, then what other thing can be wrong? 
You could add Pooling=False in the connection string, but this means a new connection is created each time.  
+ "User Id=myusername;Password=mypass;Pooling=False;";
Take a look at this article, it might help with your issue. Also, take a look at this website page, specifically the Using Connection Pooling section
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