inheritance polymorphism with static members

Question

Hi all Am I right in thinking that C# doesn't include polymorphism for static variables? Here's an example - lets say I want a supertype to define a method that prints out the name of the class. So I create the following: class Parent { static protected string className; static Parent() { className = Parent; } static public void OutputClassName() { System.Console.Out.WriteLine( className ); } } class ChildA : Parent { static ChildA() { className = ChildA; } } class ChildB : Parent { static ChildB() { className = ChildB; } } And I run the following code: Parent.OutputClassName(); ChildA.OutputClassName(); ChildB.OutputClassName(); Now I'd expect the output to be Parent ChildA ChildB but it's actually Parent Parent Parent So it seems to me that when OutputClassName is invoked and accesses the static className variable it doesn't treat this variable polymorphically e.g. the variable's value doesn't change depending on what class is used to invoke OutputClassName. When I step through the above code in a debugger the derived class's static ctors are never called - even if ChildA.OutputClassN ame() is the first method called. Is this what is meant when MS say that static constructors are not inherited? I'd love for someone to explain what's going on here and if there's a nice elegant way to implement functionality similar to above. TIA Pete

Administrator · Answer

> Inheritance is something that occurs only in instances of objects That doesn't seem right to me - if C# only supported inheritance through objects and not classes then wouldn't you have to define the subclass with reference to an instance and not a class name? In traditional OOP inheritance is at the Class level. I think instead the problem above is that the OutputClassName() method is defined in Parent and Parent's reference is to Parent's className NOT the overridden ChildA and ChildB copies. In fact I had direct experience with this in Java where I wanted to create an abstract Singleton superclass. I couldn't do it effectively (no-one has to my knowledge) because the problem is that the children's identities are never known to the parent class so you can't write the controls around instantiation up there. I tried again to do it with reflection but the overhead and klunkiness it introduced wasn't worth it. Anyway an easier way to understand this is that just because Parent has a static variable doesn't mean that the children reference it. What it means is that the children will BE a copy of the parent class and its members will also be copies. If you only have one or two methods like this just override the Parent's method and be done with it :-) Scott

Administrator · Answer

As I understand Static Variables in Microsoft's .NET you cannot implicitly inherit static variables. Inheritance is something that occurs only in instances of objects. For every instance you may inherit methods and properties of the parent instance. Static variables exist outside the instances. However you may be able to reference a private static variable if you inherite from the parent class who owns the static variable... At anyrate I can say with certainty if you have a public static variable in your parent class then you can reference it like the following Class myParent { public static string myStaticVar = Parent; } Class myChildA { public myChildA { myParent.myStaticVar = Child; } } you must include the object that you are calling the static variable from since the static varaibale sits in memory in a different part of the stack from the instance of the objects.

Administrator · Answer

Well I see one problem right off. The ctors for the child classes aren't called because you are not instantiating an instance of the child class. ClassName can never be set by the child class by calling OutputClassName statically. Bill F