What are some good methods of implementing RTS-style Box selection?

Question

I'm scouting around for different methods of implementing a typical RTS box selection. This is for 3D game - so I'm looking for methods for finding units within selection box in 3D world. Here is a quick screenshot of what a box selection looks like in 2D Starcraft:

Answer 1

Your question was vague, but the answer to both interpretations could be simple.

Heres some pseudo code:

vec2 original_xy = get_mouse_position();
vec2 new_xy;

while (get_mouse_click_state() == MOUSE_DOWN) {
        new_xy = get_mouse_position();
        render_box(original_xy, new_xy);
        ...
        context_swap_buffers();
}

vec2 a = mouse_to_world(original_xy);
vec2 b = mouse_to_world(new_xy),

const float max_height = 100.f; // maximum search height
BoundingBox aabb(vec3(min(a.x, b.x), min(a.y, b.y), 0), vec3(max(a.x, b.x), max(a.y, b.y), max_height));
List<Object> objects_in_box;

for Object x in all_objects do {
    if (x.collides(aabb)) {
        list.add(x);
    }    
}