No known instance method for selector 'valueForKey:' for id <NSObject, NSCoding> object

Question

I'm getting an error I can't shake. When I try to access an id <NSObject, NSCoding>'s property with valueForKey the code is fine for the function parameter, item, but is not for a similar id I retrieve out of an array. Why is this?

- (void) insertItem:(id<NSCoding, NSObject >) item {
            for (int i = 0; i < [self.sortDescriptors count]; i++) {
                NSString  *propertyString = self.sortDescriptors[i].key;
                int idx = 0;
                id<NSCoding, NSObject > comparisonItem = self.items[idx];
                while ([[item valueForKey: propertyString] compare:[comparisonItem valueForKey: propertyString]] == NSOrderedAscending) {
                     ....

It's this call:

[comparisonItem valueForKey: propertyString]

That is producing this error:

No known instance method for selector 'valueForKey:' 

What am I missing and how can I do the comparison for a dynamically named property?

Answer 1

Instead of id<NSCoding, NSObject >, type this value as an NSObject*.