I have a class category declared and compiled in a mac os static lib: #import <Foundation/Foundation.h> @interface NSNumber(MyExtension) -(NSString *)CallMe; @end then this staticLib is added to a mac console app in XCode and used: #import <Foundation/Foundation.h> #import "MacStaticLib.h" int main(int argc, const char * argv[]) { @autoreleasepool { NSNumber *one = [NSNumber numberWithInt:12]; NSLog(@"%@",[one CallMe]); } return 0; } Compiles fine but fails in runtime, unrecognized selector 'CallMe' send to instance... what's wrong or what I missed? Thanks!!