I saw your post and decided to dig the law up again to make sure.
I am not a lawyer: The law regarding posssession of a knife only applies if you intend to use it unlawfully against another.
See DC ST § 22-4514 (Actually, all of Title 22, Chapter 45 pertains to weapons)
(b) No person shall within the District of Columbia possess, with intent to use unlawfully against another, an imitation pistol, or a dagger, dirk, razor, stiletto, or knife with a blade longer than 3 inches, or other dangerous weapon.
I'm glad I looked this up because I misread it a long time ago and thought that my knife had to be less than 3". Since I do not intend to use it unlawfully against another, it seems that does not apply to me.
If only the law about possession of a firearm had the same common sense exemption... but hey, if everyone in D.C. who threatened a common man's life were shot and killed instead, the... hmm, my sarcasm generator has got nothin. I guess then it would be a nice place to visit.
I am not a lawyer: You may also possess pepper spray for self defense, but it must meet these specifications:
A mixture of a lacrimator including chloroacetophenone, alphacloracetophenone, phenylchloromethylketone, orthochlorobenazalm-alononitrile or oleoresin capsicum.
I'm not a chemist either, but I think that probably rules out any pepper spray products that contain tear gas... heck, maybe even dyes.
There's a requirement to register the spray at the time of purchase, but it says nothing about spray that you have not purchased in D.C. I don't know if one could construe this to mean that it is not legal to possess the pepper spray if it is not registered with the Metropolitan Police Department. My position would be that you may possess unregistered pepper spray because 1) the law only requires registration at the time of purchase and 2) since you are not a vendor, you are not required to forward the registration form to the MPD. See § 7-2502.14 for yourself.
All relevant pepper spray laws are: DC ST § 7-2502.12, DC ST § 7-2502.13, and DC ST § 7-2502.14.