Platzieren der Komponente verweigern.

[Mystic]

Hab das jetzt so realisiert:

Delphi-Quellcode:

constructor TLayeredForm.Create(AOwner: TComponent);
  var i: Word; c: TComponent;
begin
  inherited;
  if csDesigning in ComponentState then begin
    c := Owner;
    while not c.InheritsFrom(TCustomForm) do begin
      c := c.Owner;
      if c = nil then begin
        raise Exception.Create('The LayeredForm component must be put on a form!');
        self.Destroy;
        exit;
      end;
    end;
    Owner := c;
    for i := 0 to Owner.ComponentCount -1 do
      if (Owner.Components[i].InheritsFrom(TLayeredForm)) and (Owner.Components[i] <> self) then begin
        raise Exception.Create('There can be only one LayeredForm component on every form!');
        self.Destroy;
        exit;
      end;
  end;
end;