Implementing an interface in a base and derived class

Question

In C#, can you give a good example of why you would implement an interface on a base class and re-implement that interface on a derived class, rather than making the base class methods virtual.

For example:

interface IMakesNoise
{
  void Speak();
}

class Cat : IMakesNoise
{
  public void Speak()
  {
    Console.WriteLine("MEOW");
  }
}

class Lion : Cat, IMakesNoise
{
  public new void Speak()
  {
    Console.WriteLine("ROAR");
  }
}

To test the behavior:

Cat cat = new Cat();
Cat lion = new Lion();

// Non virtual calls, acts as expected    
cat.Speak();
lion.Speak();

// Grabbing the interface out is 'virtual' in that it grabs the most derived interface implementation
(cat as IMakesNoise).Speak();
(lion as IMakesNoise).Speak();

This will print out:

MEOW
MEOW
MEOW
ROAR

UPDATE: For more clarification about the why, the reason is I am implementing a compiler and I want to know the reason that C# chose this implementation of interfaces.

Answer 1

I have looked at this question.

Interface inheritance in ComVisible classes in C#

and this

C# exposing to COM - interface inheritance

As I understand, if you have two objects and want them to be visible through COM, both should explicitly inherit from the required interface.