None of those work. (Try it.) The reason is that NSMutableXXXX is a wrapper around an underlying Core Foundation "object". You'll always get success from the

None of those work. (Try it.) The reason is that NSMutableXXXX is a wrapper around an underlying Core Foundation "object". You'll always get success from the Objective C level and then an exception when you try the action from the CF level. Example SO question: http://stackoverflow.com/questions/1788690/objective-c-how-to-check-if-variable-is-nsarray-or-nsmutablearray http://stackoverflow.com/questions/1788690/objective-c-how-t... Another example discussion: http://www.cocoabuilder.com/archive/cocoa/224795-nsdictionary-mutability-test.html http://www.cocoabuilder.com/archive/cocoa/224795-nsdictionar...