Kraedin said:You need direct line-of-effect to cast most spells, magic missle included.
PHB, pg. 150 A line of effect is a straight, unblocked path that indicates what a spell can effect... [y]ou must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect.
Kraedin said:Line of effect rules apply, clairvoyance or no. The ability to see the proposed target is immaterial.