Are the people posting on this thread actual women with professional jobs in offices in DC? Because a sleeveless dress plus a cardigan is a staple office uniform for women.
Sleeveless -- can wear it all year round, including in DC summers.
I wear dresses with blazers when I have formal events or am meeting with clients, and dresses with cardigans when I am spending my day at my desk and not meeting so much with others.
OP, where do you normally shop for cardigans? Because I see them literally everywhere. All the stores.
Another +10,000. You are overthinking this OP (and many of the PPs seem to have never stepped foot in a professional DC office).
First, the jacket/blazer over a sleeveless sheath dress is practically a uniform at law firms -- PP is seriously the first person I have ever heard extoll full sleeves on dresses as somehow necessary for a professional appearance.
As for the cardigan, I think it is less about whether it is a "cardigan" and more about its fit/ style. For most dresses, it has to be well-fitted and not too long; as one of the PP's mentioned, for a fit-and-flare dress, you'd probably even want it a bit on the cropped side. I'd stay away from goofy colors- yes, to well-proportioned black cashmere cardigan, no to the fuchsia one that was always a size too big but looks OK with jeans.
I think some of the open-faced, flowing styles are good too - yes to a cut-away style or not-too-voluminous cascade, but I did not love the cocoon look, at least for a office outfit.