Question

I have an abstract class A, where I have derived the classes B and C.
Class A provides an abstract method DoJOB(), which is implemented by both derived classes.

There is a class X which has methods inside, which need to call DoJOB().
The class X may not contain any code like B.DoJOB() or C.DoJOB().

Example:

public class X
{
private A foo;

public X(A concrete)
{
foo = concrete;
}

public FunnyMethod()
{
foo.DoJOB();
}

}

While instantiating class X I want to decide which derived class (B or C) must be used.
I thought about passing an instance of B or C using the constructor of X.

X kewl = new X(new C());
kewl.FunnyMethod(); //calls C.DoJOB()

kewl = new X(new B());
kewl.FunnyMethod(); // calls B.DoJOB()

My test showed that declaring a method with a parameter A is not working. Am I missing something?
How can I implement this correctly?

(A is abstract, it cannot be instantiated)

EDIT:
Sorry, I forgot sth.

class A is a generic abstract singleton:

abstract public class A<T> where T : A<T>
{
    ....
}

public sealed class B : A<B>
{
    .....
}

public sealed class C : A<C>
{
    .....
}

See the example:
http://www.c-sharpcorner.com/UploadFile/snorrebaard/GenericSingleton11172008110419AM/GenericSingleton.aspx

Under the head line “The solution with the Generic Singleton as an abstract class”

Answer 1

For your edit:

void Main()
{
var kewl = new X<C>(new C());
kewl.FunnyMethod(); //calls C.DoJOB()

var kewl2 = new X<B>(new B());
kewl2.FunnyMethod(); // calls B.DoJOB()

}

public class X <T> where T : A<T>
{
    private A<T> foo;

    public X(A<T> concrete)
    {
        foo = concrete;
    }

    public void FunnyMethod()
    {
        foo.DoJOB();
    }
}

public abstract class A<T> where T : A<T>
{
    public abstract void DoJOB();
}

public class B : A<B>
{
    public override void DoJOB()
    {
        Console.WriteLine("B");
    }
}

public class C : A<C>
{
    public override void DoJOB()
    {
        Console.WriteLine("C");
    }
}