private void đăngToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (frmdk == null || frmdk.IsDisposed)
     {
         frmdk = new FrmDangKy();
     }
     frmdk.ShowDialog();
 }
 private void đăngToolStripMenuItem_Click(object sender, EventArgs e)
 {
     FrmDangKy frmdk = new FrmDangKy();
     frmdk.Show();
 }