Undergarments and summer time go hand in hand. Body con dresses, maxi dresses, dresses period show off all your assets especially if it is fitted. I can't speak for everyone but I know how I feel with the right undergarment on under my fitted dresses. I don't like everything moving around I want a clean smooth look.
I've been using Flexees by Madienform (their romper shapewear) and it works pretty good. It has snaps at the bottom which of course unsnaps. So I sewed the snaps shut and it's been working good since.