Question

Wondering if I am going about this the right way. I am creating a C# application that loads a few variables from my App.Config.xml file. I am loading these into a “config” class (Config.cs) along with other variables that I am loading from a MySQL database. This is what my class looks like so far:

 class Config
    {
        public static string ServerHostname = ConfigurationManager.AppSettings["ServerHostname"];
        public static string SoftwareVersion = "v0.1a";
        public static int StationID = DBConnector.GetStationID();
        public static string StationDescription = DBConnector.GetStationDescription();
        public static string StationName = ConfigurationManager.AppSettings["StationName"];
    }

I am using Config.StationName to pull the Config.StationID and Config.StationDescription from a MySQL database like this in DBConnector.cs:

public static int GetStationID()
    {
        try
        {
            MySqlConnection conn = new MySqlConnection(connStr);
            conn.Open();
            string sql = "select station_id from station_master where station_name = @station_name limit 1";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            cmd.Parameters.AddWithValue("@station_name", Config.StationName);
            object result = cmd.ExecuteScalar();
            conn.Close();
            return Convert.ToInt32(result);
        }
        catch (Exception ex)
        {
            ErrorConnection += ex.ToString();
            return 0;
        }
    }
    public static string GetStationDescription()
    {
        try
        {
            MySqlConnection conn = new MySqlConnection(connStr);
            conn.Open();
            string sql = "select station_description from station_master where station_id = '" + Config.StationID +"' limit 1";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            // cmd.Parameters.AddWithValue("@station_name", Config.StationName.ToString());
            object result = cmd.ExecuteScalar();
            conn.Close();
            MessageBox.Show(sql);
            return (result.ToString());
        }
        catch (Exception ex)
        {
            ErrorGenericDBException += ex.ToString();
            MessageBox.Show(ErrorGenericDBException);
            return "Error";
        }
    }

The DBConnector.GetStationID class works fine. It returns the int value of my station. But when I try to display the Config.StationDescription, it throws an exception of System.NullReferenceException: Object reference not set to an instance of an object at Namespace.DBConnector.GetStationDescription().

I thought that since I was using a static class for Config.StationName, i don’t have to create an instance to refer to it. I need help to understand why its throwing an exception.

Answer 1

To me it looks like you might be mixing up what part of the code has the problem. Since you’re saying that it’s GetSTationDescription that throws, I’m not sure why you then assume StationName has a problem?

I’d take a look at the line object result = cmd.ExecuteScalar();, if you put a breakpoint on the line after that, does result have a value?

Another issue is that you’re currently only closing the connection if an exception is not thrown, it would be safer to have the conn.Close(); inside a finally statement (or use a using statement so you don’t have to worry about closing it).