using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();

                SqlCommand command = new SqlCommand("SELECT realname, id, nivel FROM Users WHERE  username = @username AND password = @password", connection);
                command.Parameters.AddWithValue("@username", username);
                command.Parameters.AddWithValue("@password", password);

                SqlDataReader reader = command.ExecuteReader();

                if (reader.HasRows)
                {
                    reader.Read();
                    int id = reader.GetInt32(0);
                    string realname = reader.GetString(1);
                    string nivel = reader.GetString(2);

                    // create a new User object with the retrieved data
                    loggedInUser = new UserData(id, realname, nivel);

                    // open the main menu form
                    MainMenu mainMenu = new MainMenu(loggedInUser);
                    mainMenu.Show();
                    this.Hide();
                }
                else
                {
                    // invalid credentials, display error message
                    MessageBox.Show("Credenciais inválidas!");
                }

                reader.Close();
            }

using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();

                SqlCommand command = new SqlCommand("SELECT realname, id, nivel FROM Users WHERE  username = @username AND password = @password", connection);
                command.Parameters.AddWithValue("@username", username);
                command.Parameters.AddWithValue("@password", password);

                SqlDataReader reader = command.ExecuteReader();

                if (reader.HasRows)
                {
                    reader.Read();
                    int id = reader.GetInt32(0);
                    string realname = reader.GetString(1);
                    string nivel = reader.GetString(2);

                    // create a new User object with the retrieved data
                    loggedInUser = new UserData(id, realname, nivel);

                    // open the main menu form
                    MainMenu mainMenu = new MainMenu(loggedInUser);
                    mainMenu.Show();
                    this.Hide();
                }
                else
                {
                    // invalid credentials, display error message
                    MessageBox.Show("Credenciais inválidas!");
                }

                reader.Close();
            }

private UserData loggedInUser;
        public MainMenu(UserData user)
        {
            InitializeComponent();
            loggedInUser = user;

            // display the logged-in user's first name on the lblNomeUtilizador label
            lblNomeUtilizador.Text = loggedInUser.RealName;
        }

private UserData loggedInUser;
        public MainMenu(UserData user)
        {
            InitializeComponent();
            loggedInUser = user;

            // display the logged-in user's first name on the lblNomeUtilizador label
            lblNomeUtilizador.Text = loggedInUser.RealName;
        }

public class UserData
    {
        public int UserId { get; set; }
        public string RealName { get; set; }
        public string Nivel { get; set; }

        public UserData(int id, string realname, string nivel)
        {
            UserId = id;
            RealName = realname;
            Nivel = nivel;
        }

    }

public class UserData
    {
        public int UserId { get; set; }
        public string RealName { get; set; }
        public string Nivel { get; set; }

        public UserData(int id, string realname, string nivel)
        {
            UserId = id;
            RealName = realname;
            Nivel = nivel;
        }

    }