how do i refer to the derived type from the base class?

Question

is there a keyword for the question "?" mark below or a way to achieve the same effect without using templates?

abstract class A
{
    public abstract void Attach(? x);
}

class B : A
{
    public override void Attach(B b) {}
}

class C : A
{
    public override void Attach(C c) {}
}

so that:

var b1 = new B();
var b2 = new B();

var c = new C();

b1.Attach(b2);
b1.Attach(c); // should not compile

EDIT: with templates i mean type parameters such as Attach<T>(T x, T y) // if we ignore that the example takes 1 argument

Answer 1

Annoyingly, no. The closest you can get is:

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

class B : A<B>
{
    public override void Attach(B b) { }
}

class C : A<C>
{
    public override void Attach(C c) { }
}

This doesn't however stop someone from writing:

class D : A<B>
{
    ...
}

If you want to avoid this, you need a runtime check for this.GetType() == typeof(T) or similar in A 's constructor.

Answer 2

You can make A Generic like so:

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

class B : A<B>
{
    public override void Attach(B b) {}
}

class C : A<C>
{
    public override void Attach(C c) {}
}

Than the following does not comile

b1.Attach(c); // should not compile