Question

Strange things happens to me, I try to create mutable dictionary, but it return an immutable.

NSMutableDictionary * d = [NSMutableDictionary dictionary];
[d setObject:value forKey:key];

The d NSMutableDictionary is still NSDictionary, see in screenshot. And, of course, application crashes then executing [d setObject:value for:key]

Answer 1

Your problem is that your value is nil, and you can’t set a nil value into a dictionary.

Don’t worry about the __NSCFDictionary * and NSDictionary stuff that you see in the variable inspector. You most definitely have a mutable dictionary. What you see in the inspector is artifacts of how NSMutableDictionary is implemented (as a class cluster).