Problem:How could I show dialog before current window?

Question

 

When I use form's ShowDialog() , sometime the dialog could be hidden by current window , how could I solve this problem , thanks very much !

 _frmMessage.dShowMode = 1; //show confirm
   _frmMessage.Activate();
   DialogResult dr = _frmMessage.ShowDialog();
   _frmMessage.Visible = false;