I was wondering if this was possible. I was wondering if i could make a small long block go through a larger (but thinner) block, and then in-game do something similar to the CSG-subtract command in wed. But I would like to do this at runtime with code. Is it possible?
(It would be great to be able to make holes in walls at runtime!)

i think that is possible, but if do holes using this method you must use very fast computer, you can just imagine how many solid block you can get if crate few holes.

I thought CSG Subtract dosn't create more blocks. Wouldn't it just be the same block with a few more faces (for the new shape)?

I have been looking at it in the manual, and in the section on about the wed edit menu it says:



Anyway, does anyone know if this can be done during the game (at runtime) with code?

sure, it can be done but it isn't that easy. you could use a library like carve-csg.

if there's any other way that you could break the wall (for example: assemble it from 9 block models) I'd advice you to use this. Even non-runtime, people where always advicing to use csg subtract very sparely (can create small thin surfaces, can get your blocks pretty messed up, doesn't look too good). So at runtim I'd try not to use it if I were you.
That being said, if your object that has to cut into the wall isn't just a normal block but a more complex shape, you could create still create the wall from multiple blocks and then replace the block where the "cutter" hits the wall with a block that has already been carved into, then it yould look like the cutter just cut into the wall. If your cutter can hit the wall at different angles, however, that's not an option...