Proposal: Abstract interface

[lachbaer]

During discussing #9132 I had the idea to propose abstract interface

public abstract interface IAnimal {
    void Walk();
    int Legs { get; set; }
}

When you use an abstract interface the compiler can check at compile time if the handed class instance implements all of the methods, events and properties of it. You can then pass any class that fits.

public class Cat {
    void Walk();
    int Legs { get; set; }
    void Jump();
}

public void Go(IAnimal animal) {
  animal.Walk();
}

public static void Main() {
  var myCat = new Cat(); 
  Go(myCat); // compiler checks if myCat fulfills the IAnimal abstract contract
}

In case of a cat that has paws instead of legs I think it would be better to stick with the current standard and define a derived class with an corresponding copy constructor

public class CatWithLegs : Cat {
  public CatWithLegs(Cat _originalCat) { /* copy */ }
  public int Legs {
    get => base.Paws; // 7881
    set => base.Paws = value;
  }
}

It should not be possible to 'derive' from an abstract interface, like with an abstract class definition, just to stay consistent.

[gafter]

I think proposing a concrete syntax for a different issue is best done inside that issue's discussion thread. Closing as a dup of #9132.