What is the use of 'abstract override' in C#?

Interestingly enough, the Roslyn version of the C# compiler has an abstract override method in it, which I found odd enough to write an article about:

http://ericlippert.com/2011/02/07/strange-but-legal/

There's a useful example for this on Microsoft Docs - basically you can force a derived class to provide a new implementation for a method.

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

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

public class F : E
{
    public override void DoWork(int i)
    {
        // New implementation.
    }
}

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 find it really useful for ensuring proper ToString() implementation in derived classes. Let's say you have abstract base class, and you really want all derived classes to define meanigful ToString() implementation because you are actively using it. You can do it very elegantly with abstract override:

public abstract class Base
{
    public abstract override string ToString();
}

It is a clear signal to implementers that ToString() will be used in base class in some way (like writing output to user). Normally, they would not think about defining this override.