I already have my multiple-entry Schengen visa.
When I applied, I gave them the hotel reservation from booking.com (not paid) and they granted the visa.
But now my friend invited me to stay at his home so I canceled my reservation after having the visa. There will be any problem with the control?
No, there is no stipulation that you must stay at the same hotel reservation for which you have shown in the visa application.
However, it is best you carry a proof that you will be staying at your friend's home (for example, a letter from him addressed to the embassy which issued the visa) would work along with their pertinent details (name, address, permanent resident number/id number/etc.)
This is in case the border officers ask for proof of accommodation. Keep in mind that a visa is not a guarantee of entry, it is just a permit to request entry at the border - so the immigration officer is well within their rights to deny you entry even if you have a "valid" visa.
