c# RTS style snapping of game objects

Hi I am having trouble building a snapping system. The idea is the user selects a 3d worldspaced gameObject and drags it around the screenspace and if it passes over a gameObjects bounds in worldspace then it will snap to that gameObject in worldspace when released.

What is the best way to go about this? I am not having any luck so far. I am currently using Actions, Interfaces and Classes e.g. Igrabbable, Isnappable.

Any ideas, if you need more info let me know.