public partial class Login : Form
    {
        private readonly IAuthenticationRepository _authenticationRepository;
        private int _failedLoginCounter = 0;

        public Login()
        {
            InitializeComponent();

            // Initialize our repository
            _authenticationRepository = new AuthenticationRepository();
        }

        private void btnLogin_Click(object sender, EventArgs e)
        {
            var username = txtUsername.Text;
            var password = txtPassword.Text;

            if (string.IsNullOrEmpty(username))
            {
                MessageBox.Show("Please type your Username");
                txtUsername.Focus();
                return;
            }

            if (string.IsNullOrEmpty(password))
            {
                MessageBox.Show("Please type your Password");
                txtPassword.Focus();
                return;
            }

            // Seperate the login check and make it lously coupled from the UI (= do not refer to the UI elements, instead pass the values to a method)
            CheckLogin(username, password);
        }

        private void CheckLogin(string username, string password)
        {
            try
            {
                string errorMessage;

                // We seperated our database code in a different class
                // We can now easily call the Login method from anywhere...
                // This piece of code now is reusable.
                var isAuthenticated = _authenticationRepository.Login(username, password, out errorMessage);

                if (isAuthenticated)
                {
                    MessageBox.Show("Login Successful");
                    Hide();
                }
                else
                {
                    MessageBox.Show(errorMessage);
                    _failedLoginCounter++;
                }


                // If the user has 3 failed login attempts on a row => close.
                if (_failedLoginCounter >= 3)
                    Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }


    public interface IAuthenticationRepository
    {
        /// <summary>
        /// Tries to login using the specified Username and Password
        /// </summary>
        /// <param name="username">The username of the user who wants to login</param>
        /// <param name="password">The password of the user who wants to login</param>
        /// <param name="errorMessage">The errormessage will contain information when authentication has failed</param>
        /// <returns>Return true if authentication was succesful, false if there where any problems.</returns>
        bool Login(string username, string password, out string errorMessage);
    }

    public class AuthenticationRepository : IAuthenticationRepository
    {
        private readonly string _connectionString;

        public AuthenticationRepository()
            : this(@"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\...\Auth_Credentials.accdb;")
        {
        }

        public AuthenticationRepository(string connectionString)
        {
            _connectionString = connectionString;
        }

        public bool Login(string username, string password, out string errorMessage)
        {
            // Give out parameter a default value
            errorMessage = string.Empty;

            var numberOrResults = 0;

            // You need to use a using statement since OleDbConnection implements IDisposable
            // more inf: http://msdn.microsoft.com/en-us/library/system.data.oledb.oledbconnection(v=vs.110).aspx
            using (OleDbConnection conDataBase = new OleDbConnection(_connectionString))
            {
                // You need to use a using statement since OleDbCommand implements IDisposable
                // more info: http://msdn.microsoft.com/en-us/library/system.data.oledb.oledbcommand(v=vs.110).aspx
                using (OleDbCommand cmdDataBase = conDataBase.CreateCommand())
                {
                    cmdDataBase.CommandText =
                        "SELECT * FROM Auth_Credentials WHERE Username=@username AND Password = @password";

                    cmdDataBase.Parameters.AddRange(new OleDbParameter[]
                    {
                        new OleDbParameter("@username", username),
                        new OleDbParameter("@password", password)
                    });

                    // Open database if not open
                    if (conDataBase.State != ConnectionState.Open)
                        conDataBase.Open();

                    // You need to use a using statement since OleDbDataReader inherits DbDataReader which implements IDisposable
                    // more info: http://msdn.microsoft.com/en-us/library/system.data.common.dbdatareader(v=vs.110).aspx
                    using (OleDbDataReader myReader = cmdDataBase.ExecuteReader())
                    {
                        while (myReader != null && myReader.Read())
                        {
                            numberOrResults++;
                        }
                    }
                }
            }

            if (numberOrResults > 0)
                errorMessage = "Duplicate Username or Password";
            else if (numberOrResults == 0)
                errorMessage = "Username or Password do not match";

            return numberOrResults == 1;
        }
    }