It is consistent with how other .Net libraries work (particularly the string ones). It also works fine on null as long as the type of null is string.

It works because it's an extension method, but aren't extension methods supposed to make things nice and consistent by making it look as if you're calling an instance method?

Since this would be completely broken if it were an instance method, the reader might be surprised at first until they realize it's an extension method. So all I'm saying is that it's no less "counter-intuitive" than the normal string.IsNullOrWhiteSpace.

Indeed, the fact that extension methods happen to work with `this == null` is more of a bonus feature than something that should be deliberately used.