Do I have to disclose an arrest (assault) in Canada even if it resulted in a non conviction 12 years ago?

Who are you wanting to disclose it to? I would definitely tell my future husband about it, but I don't know anyone else who really needs to know. If you are trying to get citizenship, that's a different story. An immigration lawyer will be able to tell you, but I don't see any real problems unless it was a conviction.

You do not have to disclose it to me, but if it's a question on an immigration form, yes. Read the question carefully.

Yes, by all means disclose it. It would be the height of folly not to do so. FIrst, there is a specific question to that point on every Immigration Application, a no answer would be a misrepresntation; second your were NOT CONVICTED ! thus there is no basis for inadmissibility or potential hamr to your Application. It happened, but every single person has, at one time in their life made an unflattering mistake. But, this is not you now. Good Luck