oledb with txt

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.OleDb;
using System.IO;
using System.Linq;
using System.Net.Mime;
using System.Security.Policy;
using System.Text;
using System.Threading.Tasks;
using WCFServiceLibrary.UserHandling;

namespace WCFServiceLibrary
{
    internal static class DatabaseHandler
    {
        private static readonly OleDbConnection _connection;

        static DatabaseHandler()
        {
            Logger.Log("Setting up connection string...", LogSeverity.Information);
            CreateTablesIfNotExists();
            var finfo = new FileInfo("users.txt");
            _connection = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source="
                + finfo.DirectoryName + ";Extended Properties='text;HDR=Yes;FMT=Delimited';");
        }

        private static void CreateTablesIfNotExists()
        {
            if (!File.Exists("users.txt"))
            {
                Logger.Log("Users table doesn't exist! I'll just create it now.", LogSeverity.Warning);
                using (var sw = File.CreateText("users.txt"))
                {
                    sw.WriteLine("username;email;passwordhash");
                    sw.Flush();
                }
            }
            if (!File.Exists("proxies.txt"))
            {
                Logger.Log("Proxies table doesn't exist! I'll just create it now.", LogSeverity.Information);
                using (var sw = File.CreateText("proxies.txt"))
                {
                    sw.WriteLine("proxy_ip");
                    sw.Flush();
                }
            }
        }

        internal static List<User> GetUsers()
        {
            Logger.Log("Getting all the registered users.", LogSeverity.Information);
            return GetUsersFromReader(ExecuteQuery("SELECT * FROM users.txt"));
        }

        private static List<User> GetUsersFromReader(IDataReader dataReader)
        {
            Logger.Log("Reading data from users.", LogSeverity.Information);
            var users = new List<User>();
            while (dataReader.Read())
            {
                users.Add(new User
                {
                    Username = dataReader.GetString(0),
                    EmailAddress = dataReader.GetString(1),
                    PasswordHash = dataReader.GetString(2)
                });
            }
            dataReader.Close();
            return users;
        }

        private static IDataReader ExecuteQuery(string query)
        {
            Logger.Log("Executing SQL query: " + query, LogSeverity.Information);
            var cmd = new OleDbCommand(query, _connection);
            OpenConnection();
            var result = cmd.ExecuteReader(CommandBehavior.CloseConnection);
            return result;
        }

        private static void OpenConnection()
        {
            if (_connection.State == ConnectionState.Closed) _connection.Open();
        }

        private static int ExecuteNonQuery(OleDbCommand cmd)
        {
            Logger.Log("Executing SQL command: " + cmd.CommandText, LogSeverity.Information);
            cmd.Connection = _connection;
            OpenConnection();
            var result = cmd.ExecuteNonQuery();
            _connection.Close();
            return result;
        }

        internal static RegisterResult AddUser(User user)
        {
            Logger.Log("Adding new user to database by the name of " + user.Username, LogSeverity.Information);
            var cmd = new OleDbCommand("INSERT INTO [users.txt] VALUES (@username,@email,@password)", _connection);
            cmd.Parameters.Add("@username", OleDbType.BSTR).Value = user.Username;
            cmd.Parameters.Add("@email", OleDbType.BSTR).Value = user.EmailAddress;
            cmd.Parameters.Add("@password", OleDbType.BSTR).Value = user.PasswordHash;

            var affectedRows = ExecuteNonQuery(cmd);
            return affectedRows > 0
                ? new RegisterResult { Success = true }
                : new RegisterResult { Error = "Failed to register user!" };
        }

        internal static ProxyResult AddProxy(Proxy proxy)
        {
            Logger.Log("Adding new proxy to the database by the IP of " + proxy.ProxyIp, LogSeverity.Information);
            var cmd = new OleDbCommand("INSERT INTO [proxies.txt] VALUES (@proxy_ip, @port, @responsetime)");
            cmd.Parameters.Add("@proxy_ip", OleDbType.BSTR).Value = proxy.ProxyIp;
            cmd.Parameters.Add("@port", OleDbType.Integer).Value = proxy.Port;
            cmd.Parameters.Add("@responsetime", OleDbType.Integer).Value = proxy.ResponseTime;

            var affectedRows = ExecuteNonQuery(cmd);
            return affectedRows > 0
                ? new ProxyResult { Success = true }
                : new ProxyResult { Error = "Failed to insert proxy!" };
        }

        internal static List<Proxy> GetProxies()
        {
            Logger.Log("Getting all the proxies.", LogSeverity.Information);
            return GetProxiesFromReader(ExecuteQuery("SELECT * FROM proxies.txt ORDER BY responsetime"));
        }

        // na ez az ami nem működik, mert az oledb révén a fájl használatban van.
        // viszont az oledb mezei egyszerűséggel nem támogatja a delete utasítást ...
        internal static void RemoveProxy(Proxy proxy)
        {
            var proxies = File.ReadAllLines("proxies.txt").ToList();
            for (int i = 0; i < proxies.Count; i++)
            {
                if (proxies[i].Contains(proxy.ProxyIp))
                {
                    proxies.RemoveAt(i);
                    break;
                }
            }
            File.WriteAllLines("proxies.txt", proxies);
        }

        private static List<Proxy> GetProxiesFromReader(IDataReader dataReader)
        {
            Logger.Log("Reading data from proxies.", LogSeverity.Information);
            var proxies = new List<Proxy>();
            while (dataReader.Read())
            {
                proxies.Add(new Proxy(dataReader.GetString(0), int.Parse(dataReader.GetString(1)), int.Parse(dataReader.GetString(2))));
            }
            return proxies;
        }
    }
}