Access property of a subcomp through expressions.

Question

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

Answer 1

Thanks for the inputs.
Unfortunatly there is still the issue about the timing.
If I use ".value" I get the right value from the Slider from the parent but it will be executed based on the time of the parent comp. Which means the subcomp copies the time to its own "time-dimension".
For example: If I have the checkbox control(in the maincomp) on 0 until frame10 and if I key it to 1 on frame11, the opacity of the layer(in the subcomp )is going to change on frame11 within the subcomp.
If subcomp starts lets say on f5 in the maincomp the actual script would be executed on frame16 instead of 11. Therefore I need some additional techniques to let the magic happen when the checkbox-control is actually keyed. 
100*control.effect(1).param("Checkbox").value
The following pic should demonstrate it:
a) Checkbox is keyed to 0 and should hide a layer in the subcomp at that time
b) Currently the layers opacity is set to 0 here.