I want to add a CNContactPickerViewController to my app.
The user has to tap on a button and it opens up, so that he now can select a contact.
If the user selected a contact, its name will be used by my program.
My question: is it necessary to ask the user for permission using a alertView? Or can I just open up the ContactPicker?
You do not need to ask for permission when using the CNContactPickerViewController.
The CNContactPickerViewController is not a view controller handled by your app. Instead it is handled by the os itself (by opening the Contact App within your app) and therefore you cannot control the contents via code. For this reason, you do not need to request permission.
If you try to access the address book via the AddressBook framework, you will be required to request permission as you will have full access to the address book at code level.