constructor TMyIBQuery.Create (AOwner: TComponent);
begin
inherited;
// you dont need this
// if csDesigning in ComponentState then begin
SomeProperty := True;
OtherProperty := False;
// end;
end;

> This was to change the defaults of some properties and avoid
> tweaking the component every time I drop it on the form. But there
> is undesireable side effect: when I changed the property to other
> value in the designer and after a while reopened the form in the
> designer again, the constructor set the property back.

To change defaults you dont need to check for csDesigning.

In the class declaration you normally want the following...

class TMyIBQuery = class(TIB_Query)
published
SomeProperty default True;
OtherProperty default False;
end;

to stop the defaults from begin saved to the DFM unnecessarily.

and then the constructor as you had it without the designer checks.
(csDesigning is not in the ComponentState when the component is
executed at runtime, so the default is never set).

Note that overrides applied at designtime are applied in the "Loaded"
method which is called after construction, so the constructor code
above will not prevent you changing the properties at designtime.

PS. Deriving your own component classes is a GREAT idea. I thoroughly
recommend it for components that you use a lot - even if you dont have
anything you want to override immediately.

--
Geoff Worboys - TeamIBO
Telesis Computing