((void (*)(id, SEL, double))objc_msgSend)(window, sel_registerName("setAlphaValue:"), 1); If the method signature changes (unlikely for a system header, but po

((void (*)(id, SEL, double))objc_msgSend)(window, sel_registerName("setAlphaValue:"), 1); If the method signature changes (unlikely for a system header, but possible if you're calling into your own code) then you're hosed. Or you could just mess up and copy/paste the wrong signature, or forget to change it. Basically, it's pretty error-prone no matter what you do. Using Objective-C directly is much safer (and luckily you usually can, since ObjC is pretty much a drop-in replacement for C).