Good Case For Interfaces

The argument against them is thus: If a class is properly setup (with its public and private members) then an interface is just extra overhead because those that use the class are restricted to public members. If you need to have an interface that is implemented by more than 1 class then just setup inheritance/polymorphism.

Consider the following code:

interface ICrushable
{
  void Crush();
}

public class Vehicle
{
}

public class Animal
{
}

public class Car : Vehicle, ICrushable
{
  public void Crush()
  {
     Console.WriteLine( "Crrrrrassssh" );
  }
}

public class Gorilla : Animal, ICrushable
{
  public void Crush()
  {
     Console.WriteLine( "Sqqqquuuuish" );
  }
}

Does it make any sense at all to establish a class hierarchy that relates Animals to Vehicles even though both can be crushed by my giant crushing machine? No.

Interfaces and abstract classes model different things. You derive from a class when you have an isA relationship so the base class models something concrete. You implement an interface when your class can perform a specific set of tasks.

Think of something that's Serializable, it doesn't really make sense (from a design/modelling point of view) to have a base class called Serializable as it doesn't make sense to say something isA Serializable. Having something implement a Serializable interface makes more sense as saying 'this is something the class can do, not what the class is'