using static property of an abstract class inside a generic class that works on subclasses of the abstract class

Question

Hi, I have a question regarding the following code:

abstract class a
{
    public static string x;
}



class b<c> where c : a
{
    public void f()
    {
        c.x=10;
    }
}

This code does not compile. I get an error at the statement c.x=10; . The problem makes it look as if the condition where c:a does not have any effect at all.Can someone please explain why this is an error? Is it not true that x is shared as a static member by all children of a? And is there a way to circumvent this problem?

What I am trying to achieve is this : i have a subclass of a, all of whose objects share a common property and this property has to be set through f() in the generic class b. Is it alright if i replace the statement in question with a.x=10? If not, how is a.x different from c.x (or h.x where h is the subclass of a)?

Answer 1

Static members are not inherited, although it's confusingly possible to access a static member through a derived type. For example, in the following code

class P
{
    public static string X;
}

class Q : P { }

class R : P { }

you can access P.X through P.X or Q.X or R.X but it's still the same field:

P.X = "Hello";
Q.X = "World";
Console.WriteLine(R.X);  // prints "World"

As you've discovered, you can't do this with generic type parameters. But accessing X though the type parameter doesn't really make a lot of sense, because all you change is P.X which you write directly without the generic type parameter.

---

I'm not really sure what you're trying to achieve. If you have an abstract class A and want all instances of types that derive from A to have a certain property, you can define this:

abstract class A
{
    public abstract string X
    {
        get;
    }
}

class A1 : A
{
    public override string X
    {
        get { return "A1"; }
    }
}

class A2 : A
{
    public override string X
    {
        get { return "A2"; }
    }
}

---

If you want to associate a bit of information with a type (not instance), you can define a static field that is parameterized with a type using a generic class:

class Info<T>
{
    public static string X;
}

Info<A1>.X = "Hello";
Info<A2>.X = "World";

Console.WriteLine(Info<A1>.X);  // prints "Hello"
Console.WriteLine(Info<A2>.X);  // prints "World"

---

What about this?

abstract class Job
{
    public abstract string ExePath
    {
        get;
    }

    public void Execute(string[] args)
    {
        Console.WriteLine("Executing {0}", this.ExePath);
    }
}

abstract class Job<T> where T : Job<T>
{
    public override string ExePath
    {
        get { return JobInfo<T>.ExePath; }
    }
}

class ConcreteJob1 : Job<ConcreteJob1> { }

class ConcreteJob2 : Job<ConcreteJob1> { }

static class JobInfo<T> where T : Job<T>
{
    public static string ExePath;
}

static class JobInfoInitializer
{
    public static void InitializeExePaths()
    {
        JobInfo<ConcreteJob1>.ExePath = "calc.exe";
        JobInfo<ConcreteJob2>.ExePath = "notepad.exe";
    }
}

This matches closely the process you describe in your comment. It should work, although it's not how I would design a configurable Job model.

Answer 2

If you want a different static backing store for each subclass, but want to be able to access the state from an instance, polymorphically, you can do something like this:

abstract class A
{
    public abstract string X { get; set; }
}

class D : A
{
    private static string _x;

    public override string X
    {
        get { return _x; }
        set { _x = value; }
    }
}

Different subclasses of A can provide their own implementation of X, including using a backing static store, if this is desirable. Note that this is not necessarily a good idea, as this kind of tricky global state (pretending to be an instance property) may make your code difficult to understand and maintain.

Answer 3

This is an error because c is a generic type parameter, not a type. Static members are only accessible via types.

Is it alright if i replace the statement in question with a.x=10? If not, how is a.x different from c.x (or h.x where h is the subclass of a)?

You can indeed replace the statement with a.x=10;. As you observed, that means b.x, c.x, and d.x will all equal 10 as well.