Question

Can someone explain to me why this doesn’t work:

CoreDataClass *classObject = (CoreDataClass *)[some method that returns a dictionary with exact KVC pairs that match CoreDataClass];

NSString *myString = classObject.stringProperty;

But this does:

CoreDataClass *classObject = (CoreDataClass *)[some method that returns a dictionary with exact KVC pairs that match CoreDataClass];
NSString *myString = [classObject valueForKey:@"stringProperty"];

EDIT:
What’s the easiest way to cast the dictionary as my NSManagedObjectClass CoreDataClass so I can access properties directly?

Answer 1

Casting objects only appears to work (in the sense that you won’t get type-checking errors) because it’s a hint to the compiler, but it doesn’t actually change anything about what the pointer points to, so you are still pointing to an NSDictionary. This is because, at the end of the day, you are essentially casting a pointer to a pointer, but telling Xcode that you are allowed to send a different set of selectors to it.

For NSManagedObjects, creation from a dictionary depends on a few things, but the recommended way is to make a class method on your custom class which will use NSEntityDescription and you NSManagedObjectContext, and sets the properties from the dictionary to the object:

+(CoreDataClass *) coreDataObjectWithDictionary:(NSDictionary *) spec {
  CoreDataClass *myInstance = [NSEntityDescription insertNewObjectForEntityForName: @"CoreDataClass" inManagedObjectContext: [myMOCProvider sharedMOC];
  myInstance.someProp = [spec valueForKey:@"someProp"];
}