Question

I’m programming in C#. I’m trying to make a class, that when called will create a connection to the database.

My database connection class is here:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.OleDb;

namespace HouseServer
{

    class db
    {

        // Variable to hold the driver and location of database
        public static OleDbConnection dbConnection;

        // Database connection
        public db()
        {

            // Define the Access Database driver and the filename of the database
            dbConnection = new OleDbConnection("Provider=Microsoft.Ace.OLEDB.12.0; Persist Security Info = False; Data Source=Houses.accdb");

            // Open the connection
            dbConnection.Open();
        }
    }
}

And the main program is here:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.OleDb;

namespace HouseServer
{
    class Program : db
    {

        // List for holding loaded houses
        static List<house> houses = new List<house>();

        // Variable to hold "command" which is the query to be executed
        private static OleDbCommand query;

        // Variable to hold the data reader to manipulate data from the database
        static OleDbDataReader dataReader;

        static void Main(string[] args)
        {
            // Get the houses in a list
            List<house> c = getHousesFromDb();

            foreach (house yay in c)
            {
                // Show each house's full address
                Console.WriteLine(yay.house_number + " " + yay.street);
                Console.WriteLine(yay.house_town);
                Console.WriteLine(yay.postcode);
            }

            // Readline to prevent window from closing
            Console.ReadLine();
        }

        // Function which loads all of the houses from the database
        private static List<house> getHousesFromDb()
        {

            // Define the query to be executed
            query = new OleDbCommand("SELECT * FROM houses", dbConnection);

            // Execute the query on the database and get the data
            dataReader = query.ExecuteReader();

            // Loop through each of the houses
            while (dataReader.Read())
            {
                // Create a new house object for temporarily storing house
                house house = new house();

                // Create the house that we've just loaded
                house.house_id = Convert.ToInt32(dataReader["house_id"]);
                house.house_number = Convert.ToInt32(dataReader["house_number"]);
                house.street = dataReader["house_street"].ToString();
                house.house_town = dataReader["house_town"].ToString();
                house.postcode = dataReader["house_postcode"].ToString();

                // Now add the house to the list of houses
                houses.Add(house);
            }

            // Return all of the houses in the database as a List<house>
            return houses;
        }
    }
}

I thought that putting class Program : db would call the db constructor when the program opens, but when the code gets to the line dataReader = query.ExecuteReader();, it comes up with the error “ExecuteReader: Connection property has not been initialized.”.

All I’m trying to achieve is a database connection within another class, that I can call and have available to all of my code.

Am I supposed to call the database class in a different way?

Answer 1

No, nothing’s creating an instance of Program, and nothing’s creating an instance of db. However, I’d strongly suggest you change your design completely:

• Don’t have a static field for your database connection. Open it when you need one, use it, close it. You should very rarely need to store it in anything other than a local variable.
• Try not to use static variables at all if you can help it. They make your code harder to test, as they represent global state – that’s harder to reason about than local state. In your program, I’d use local variables entirely.
• Don’t use inheritance for this sort of thing – your Program type doesn’t logically derive from db
• Follow .NET naming conventions for methods, classes and properties. Making your code “feel” like idiomatic C# will go a long way to making it more readable for other people.