private void MoFormDangNhap()
{
frm_DangNhap m_frmDangNhap = frm_DangNhap.Instance;
m_frmDangNhap.MdiParent = this;
m_frmDangNhap.Dock = DockStyle.Fill;
m_frmDangNhap.Show();
}
private void frm_DangNhap_FormClosed(object sender, FormClosedEventArgs e)
{
_Instance = null;
}