const int WM_SYSCOMMAND = 0x112;
const int SC_CLOSE = 0xF060;
const int SC_MINIMIZE = 0xF020;
const int SC_MAXIMIZE = 0xF030;
protected override void WndProc(ref Message m)
{
  if (m.Msg == WM_SYSCOMMAND)
  {
    if (m.WParam.ToInt32() == SC_MINIMIZE)
    {
      this.Visible = false;
      return;
    }
  }
  base.WndProc(ref m);
}

private void Form1_Deactivate(object sender, EventArgs e)
{
  if (this.WindowState == FormWindowState.Minimized)
 this.Visible = false;
}