private void frmMain_Shown(object sender, EventArgs e)
{
frmDangNhap dangnhap = new frmDangNhap(this);
dangnhap.Show();
}
private void mnuDangnhap_Click(object sender, EventArgs e)
{
frmDangNhap dangnhap = new frmDangNhap(this);
dangnhap.Show();
}