Question

Can we use only NSString objects as key in an NSDictionary? How do we know which objects can be used and which cannot?

Answer 1

From the documentation:

In general, a key can be any object (provided that it conforms to the NSCopying protocol), but note that when using key-value coding the key must be a string (see “Key-Value Coding Fundamentals”).

So you can use anything copyable besides strings, but they’ll be problematic with KVC. I just use strings for keys to keep things safe, consistent and simple.