private void btnNVTroVe_Click(object sender, EventArgs e)
        {
            this.Hide();
            fBatDau f = new fBatDau();

            f.Show();
        }
        private void btnDVDxuat_Click(object sender, EventArgs e)
        {
            //btnDVDnhap.Show();
            //btnDVDki.Show();
            //btnDVDxuat.Hide();
            //labelTenKH.Text = null;
            this.Hide();
            fDangNhap.UserInformation.CurrentLoggedInUser = null;
            fBatDau f = new fBatDau();

            f.Show();
        }
        private void btnDNDnhap_Click(object sender, EventArgs e)
        {
            if (txbDNTenDN.Text == "nv" && txbDNMk.Text == "nv")
            {
                this.Hide();
                fNhanVien f = new fNhanVien();
                f.Show();
            }
            else
            {
                try
                {
                    _connection = Connection.ConnectionData();
                    string proc = "sp_DangNhap";
                    _command             = new SqlCommand(proc);
                    _command.CommandType = CommandType.StoredProcedure;
                    _command.Connection  = _connection;
                    _command.Parameters.Add("@id", SqlDbType.VarChar, 20);
                    _command.Parameters.Add("@mk", SqlDbType.VarChar, 20);
                    _command.Parameters.Add("@maKH", SqlDbType.VarChar, 20).Direction   = ParameterDirection.Output;
                    _command.Parameters.Add("@hoTen", SqlDbType.NVarChar, 30).Direction = ParameterDirection.Output;

                    _command.Parameters["@id"].Value = txbDNTenDN.Text;
                    _command.Parameters["@mk"].Value = txbDNMk.Text;

                    int    n     = _command.ExecuteNonQuery();
                    string maKH  = (string)_command.Parameters["@maKH"].Value;
                    string hoTen = (string)_command.Parameters["@hoTen"].Value;

                    MessageBox.Show("Xin chào \n" + maKH + "\n" + hoTen);
                    _connection.Close();

                    KhachHangDTO username = new KhachHangDTO(maKH, hoTen, "", "", "", "", "", "", "");
                    UserInformation.CurrentLoggedInUser = username;
                    this.Hide();
                    fBatDau f = new fBatDau();
                    f.Show();
                    _connection.Close();
                }
                catch (SqlException ex)
                {
                    MessageBox.Show(ex.Message);
                }
            }
        }
        private void fDangNhap_FormClosed(object sender, FormClosedEventArgs e)
        {
            fBatDau f = new fBatDau();

            f.Show();
        }