If a virtual method is declared 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.