using MySql.Data.MySqlClient;
using System.Windows.Forms;

namespace pControll
{
    public class Database
    {
        private string mysqlServer   = pControll.Properties.Resources.mysqlServer;
        private string mysqlDatabase = pControll.Properties.Resources.mysqlDatabase;
        private string mysqlUsername = pControll.Properties.Resources.mysqlUsername;
        private string mysqlPassword = pControll.Properties.Resources.mysqlPassword;

        private static Database mysqlInstance;
        private MySqlConnection mysqlConnection;

        private static string errorMessage;

        private Database()
        {
            mysqlConnection = createConnection();
            mysqlConnection.Open();
        }

        public static Database getMysqlInstance()
        {
            if (mysqlInstance == null)
                mysqlInstance = new Database();

            return mysqlInstance;
        }

        public MySqlConnection getMysqlConnection()
        {
            return mysqlConnection;
        }

        private MySqlConnection createConnection()
        {
            MySqlConnection connection = new MySqlConnection();
            string connectionString = "Server="+mysqlServer+"; database="+mysqlDatabase+"; UID="+mysqlUsername+"; password="+mysqlPassword;

            connection.ConnectionString = connectionString;

            return connection;
        }

        public static string getErrorMessage(int errorCode)
        {
            switch (errorCode)
            {
                case 0:
                    errorMessage = "Sikertelen csatlakozás az adatbázishoz";
                    break;
                case 1042:
                    errorMessage = "Sikertelen csatlakozás az adatbázis szerverhez";
                    break;
                default:
                    errorMessage = null;
                    break;
            }

            return errorMessage;
        }

        public static void showErorrMessage(string errorMessage)
        {
            if(errorMessage == null)
                errorMessage = "Nem definiált hibaüzenet";

            MessageBox.Show(errorMessage, "Adatbázis hiba", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }

        ~Database()
        {
            mysqlConnection.Close();
        }
    }
}

using MySql.Data.MySqlClient;
using System;
using System.Windows.Forms;

namespace pControll
{
    public partial class Form2 : childForm
    {

        public Form2()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                MySqlCommand mysqlQuery = new MySqlCommand("SELECT* FROM orders_semilab WHERE pSzam = 'P3560/1'", Database.getMysqlInstance().getMysqlConnection());
                MySqlDataReader mysqlReader = mysqlQuery.ExecuteReader();

                if (mysqlReader.Read())
                {
                    MessageBox.Show(mysqlReader["megnevezes"].ToString());
                }

                mysqlReader.Close();
            }
            catch (MySqlException ex)
            {
                Database.showErorrMessage(Database.getErrorMessage(ex.Number));
            }
        }
    }
}