Floor length is not the only answer to a black tie dress code and if you know what you are doing a cocktail length dress can look more considered than a gown. The key is that everything else has to work harder. The fabric needs to be clearly evening weight. The silhouette needs to read as formal rather than just smart. The finish and the details matter more than they do at any other length. Get those things right and a midi or knee length dress at a black tie event looks like a deliberate and confident choice rather than someone who could not find a gown in time.