Ooooh… fun question. The answer is a c-ism.
Consider:
@interface MyClass : NSObject
@end
@implementation MyClass
@end
Now, say you have:
...
MyClass *m = nil;
...
In that context, the compiler sees MyClass as a type definition. The * says that the variable m is a pointer to a hunk o' memory that contains one (or many -- don't forget your C pointer-fu) MyClass instances.
In other words, MyClass is a type.
But, in the context of something like:
[someInstance isKindOfClass: x ];
x must be an rvalue or, in human terms, the value of an expression. A type, however, cannot be used as an rvalue.
That [MyClass class] works is actually a bit of a hack, both in the language and the compiler in that the grammar specifically allows a type name to be the message receiver (to be the target of a method call).
And, as a matter of fact, you can do:
typedef MyClass Foo;
....
[MyClass class];
[Foo Class];
It’ll all work. However, you can’t do the following but the error message is illuminating:
[NSUInteger class];
error: ‘NSUInteger’ is not an Objective-C class name or alias
Now, why not special case it everywhere as a bare name?
That colludes type names and rvalues and you quickly end up having to swallow something like [foo isKindOfClass: (MyClass)]; while barfing on [foo isKindOfClass: (MyClass *)]; which then encroaches upon typecasting territory in a rather uncomfortable fashion.