Comments by bjoho (Top 3 by date)

Think of it like this; At the outset you have all points in one collection NotShadowed, WHILE there are still some points in that collection (the outer loop) iterate over those and find the ones (toBeRemoved) that are now shadowed under a new angle. Outside (after) the inner loop, remove the new shadowed points from the NotShadowed collection. This goes on WHILE there are still some points left in NotShadowed. BREAK the outer loop on some criteria if you want.

I think it is that RemainingXPoints and NotShadowedYet is the same reference (object) and the statement "NotShadowedYet.Add(XData)" in the inner loop therefore changes the collection that controls the inner loop.