using MySql.Data.MySqlClient;
...
    private void buttonOk_Click(object sender, EventArgs e)
    {
        timer2.Start();
    }

    public static string Datasource = "localhost";
    public static string Port = "3306";
    public static string InitialCatalog = "user";
    public static string Username = "user";
    public static string Password = "pass";

    //MySql
    MySqlConnection connection = new MySqlConnection("datasource=" + Datasource + ";port=" + Port + ";Initial Catalog='" + InitialCatalog + "';username=" + Username + ";password=" + Password);
    MySqlDataAdapter adapter;
    DataTable table = new DataTable();

    public void MySqlConnect()
    {
        adapter = new MySqlDataAdapter("SELECT `username`, `password` FROM `users` WHERE `username` = '" + textBoxUsername.Text + "' AND `password` = '" + textBoxPassword.Text + "'", connection);
        adapter.Fill(table); //PROBLEM

        if (table.Rows.Count <= 0)
        {
            panel1.Height = 0;
            labelMessage.ForeColor = Color.Red;
            labelMessage.Text = "Username Or Password Are Invalid";
            timer1.Start();
        }
        else
        {
            panel1.Height = 0;
            labelMessage.ForeColor = Color.Green;
            labelMessage.Text = "Login Successfully";
            timer1.Start();
        }

        table.Clear();
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        if (panel1.Height != 100)
        {
            panel1.Height = panel1.Height + 5;
            if (panel1.Height == 100)
            {
                timer1.Stop();
            }
        }
    }

    private void timer2_Tick(object sender, EventArgs e)
    {
        if (panel1.Height != 0)
        {
            panel1.Height = panel1.Height - 5;
            if (panel1.Height == 0)
            {
                timer2.Stop();
            }
        }
    }

    private void checkBoxShowPass_CheckedChanged(object sender, EventArgs e)
    {
        if (checkBoxShowPass.Checked)
        {
            textBoxPassword.UseSystemPasswordChar = true;
        }
        else
        {
            textBoxPassword.UseSystemPasswordChar = false;
        }
    }
...