Extending Curved Surface.

I am looking to acheive the same effect as the "extend surface" commnad in Rhino and have not had any luck so far with it in GH. 

So Im trying to use it in VB...which I know very little about. 

I think that I have all the other parameters definded except for the picking the edge. Does anyone have any isight into this 

Any help is appriciated. 





Thanks, 

Neal