This is all I could find so it might be out of date, and I couldn't find any carry rules.
I edited it for the relevant bits:
Notice the bit about "intent". If it wasn't for intent every kitchen knife would have to be less than 3", or ever resterant and homeowner with kitchen knifes would be in violation.
I edited it for the relevant bits:
District of Columbia - § 22-4514.
Possession of certain dangerous weapons prohibited; exceptions.
(b) No person shall within the District of Columbia possess, with intent to use unlawfully against another, .... a knife with a blade longer than 3 inches, or other dangerous weapon.
Notice the bit about "intent". If it wasn't for intent every kitchen knife would have to be less than 3", or ever resterant and homeowner with kitchen knifes would be in violation.