如果虚拟方法被声明为abstract

Question

My friend asks me if an abstract method could have virtual modifier. And I said, No . Because, an abstract method is implicitly also a virtual method, it cannot have the modifier virtual.

But while reading one of the MSDN articles , I have seen this:

... If a virtual method is declared abstract , it is still virtual to any class inheriting from the abstract class. A class inheriting an abstract method cannot access the original implementation of the method—in the previous example, DoWork on class F cannot call DoWork on class D. In this way, an abstract class can force derived classes to provide new method implementations for virtual methods....

I can't understand first sentence correctly. Could you please, explain me what they wants to say?

Thanks.

Answer 1

It becomes clearer when you look at the code example directly above the quoted paragraph:

public class D
{
    public virtual void DoWork(int i)
    {
        // Original implementation.
    }
}

public abstract class E : D
{
    public abstract override void DoWork(int i);
}

The virtual method D.DoWork is inherited by E , and, there, declared abstract. The method is still virtual , it has just become abstract as well .

As you correctly state, an abstract method is always virtual. If your friend is still unconvinced, here's an official quote for that:

An abstract method is implicitly a virtual method.

Answer 2

Abstract classes may override virtual members with abstract ones:

public class B
{
    public virtual void M() { }
}
public abstract class D : B
{
    public abstract override void M();
}
public abstract class D2 : D
{
    public override void M() { }
}

The sentence says that D2 must override void M() , because it is declared abstract in D . If it were declared as D2 : B , this would be optional, but as it stands, D2 has to comply with the contracts specified in D , but M() will also behave like any other member overriding a "normal" virtual member, since M() is both virtual and abstract.