5 public interface IBase {
9 public interface IDerived : IBase {
12 public class Base : IBase {
16 public virtual int Do() {
21 public class Derived : Base, IDerived {
26 static int Main(string[] args)
28 IDerived id = new Derived();
31 IBase ib = (IBase) id;
34 Derived d = (Derived) id;