Access property of a subcomp through expressions.

I got a main composition. It got a subcomposition and a null-object(with a control checkbox).

How can I access a layers opacity in the subcomposition throught an expression in the main composition?

The goal would be to reuse the subcomp. Once with a layer visible and another time without.

So far I wrote this in the control checkbox:

if(value == 1)

{

comp("subcomp").layer("xy").opacity =100 ;

}

else

{

comp("subcomp").layer("xy").opacity =0 ;

}

I tried also to write an expression directly in the going to be hidden layers opacity:

var control  = comp("maincomp").layer("control");

100*control.effect(1).param("SliderControl").valueAtTime(control.time) ;

This actually worked when I toggled the control checkbox manually, but as soon I keyed it, it didnt work anymore.

It was kind of a time conversion problem: It takes the time of the parent but applies it to the subcomp……

Thanks for help