C# static member "inheritance" - why does this exist at all?

Answer 1

That's how it works, would probably just be a stupid answer in most cases. But in this case, it is how it works; since you derive from A you say that you are A + the extra features you add.

Therefore you need to be able to access the same variables that you would through an instance of A.

However, inheriting a static class makes no sense while access to the static members / fields / methods does.

An example of this is the following:

internal class BaseUser
{
    public static string DefaultUserPool { get; set; }
}
internal class User : BaseUser
{
    public int Id { get; set; }
    public string Name { get; set; }
    public User Parent { get; set; }
}

Where the test looks like this:

User.DefaultUserPool = "Test";
BaseUser.DefaultUserPool = "Second Test";

Console.WriteLine(User.DefaultUserPool);
Console.WriteLine(BaseUser.DefaultUserPool);

Both of the WriteLines outputs "Second Test", this is because both BaseUser and User should use DefaultUserPool, by design. And overriding static implemented methods wouldn't make mucn sense since it's just an accessor in the child-class.

There can be only one. Overriding it would mean that there's a new implementation for that sub-class, which would kill the term "static".

Answer 2

I think it's for accessing protected static members of the base class.

class Base
{
    protected static void Helper(string s)
    {
       Console.WriteLine(s);
    }
}

class Subclass : Base
{
   public void Run()
    {
       Helper("From the subclass");
    }
}

Answer 3

Actually, as I understand it, this is just a shortcut provided by the compiler. Syntax sugar. B.M() will just compile to A.M() since B does not have a static M() and A does. It's for easier writing, nothing else. There is no "static inheritance".

Added: And the requirement for new when "redefining" is just so that you don't accidentally shoot yourself in the foot.

Answer 4

I rather have access to all my based static members in derived classes. Otherwise i would need to know exactly where the static member was defined and call it explicitly.

When using Intellisense you can automatically know every static member available to that kind of class.

Of course, they are not inherited, it's just a shortcut

Answer 5

So, the "inheritance" of static members merely looks like namespace pollution

That's right, except that one guy's pollution is another guy's added spicy flavouring.

I think Martin Fowler, in his work on DSLs, has suggested using inheritance in this way to allow convenient access to static methods, allowing those methods to be used without class name qualification. So the calling code has to be in a class that inherits the class in which the methods are defined. (I think it's a rotten idea.)

In my opinion, static members should not be mixed into a class with a non-static purpose, and the issue you raise here is part of the reason why it's important not to mix them.

Hiding private static mutable data inside the implementation of an otherwise "instancey" class is particularly horrible. But then there are static methods, which are even worse mixers. Here's a typical use of static methods mixed into a class:

public class Thing
{
    // typical per-instance stuff
    int _member1;
    protected virtual void Foo() { ... }
    public void Bar() { ... }

    // factory method
    public static Thing Make()
    {
        return new Thing();
    }
}

It's the static factory method pattern. It's pointless most of the time, but even worse is that now we have this:

public class AnotherThing : Thing { }

This now has a static Make method which returns a Thing, not a AnotherThing.

This kind of mismatch strongly implies that anything with static methods should be sealed. Static members fail to integrate well with inheritance. It makes no sense to have them heritable. So I keep static things in separate static classes, and I gripe about redundantly having to declare every member static when I've already said that the class is static.

But it's just one of those too-late-now things. All real, working languages (and libraries, and products) have a few of them. C# has remarkably few.

Answer 6

So... What's the alternative?

The question mentions...

why doesn't the compiler report an error? After all, there's no "M" method in "B"...

But there is a derived "M" method in "B" class.

If the compiler did not present the programmer a unified virtual table for base cases, then the programmer would have to go hunting through base types to find static methods. This would break polymorphism.

Wikipedia...

Subtype polymorphism, almost universally called just polymorphism in the context of object-oriented programming, is the ability of one type, A, to appear as and be used like another type, B....

In strongly typed languages, polymorphism usually means that type A somehow derives from type B, or type C implements an interface that represents type B.