namespace MyApplication {
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            var f = new LoginForm();

            if (DialogResult.OK == f.ShowDialog(null))
            {
                Application.Run(new Form1());
            }
        }
    }
}

private void buttonEnter_Click(object sender, EventArgs e)
{
    if (textBoxUserName.Text == sUserName & textBoxPassword.Text == sUserPassword)
    {
        this.DialogResult = DialogResult.OK;
        this.Close();
    }
    else
    {
        MessageBox.Show("Wrong user name or password!", "Login");
    }
}