Dear SolidWorks experts,

I have an "L" shaped sketch. I would like to extrude it, up until a certain sketch line.
And whenever position of that sketch line changes, I would like that the extrusion length changes as well.
Is this possible?

When I open the "Extrude" dialog, all sketches (apart from parent "L" sketch) suddenly disappear:
 

What am I doing wrong?
I would be very grateful for any kind of help.