public interface I1
{
	void test();
}
public class C1: I1
{
	public virtual void test()
	{
		System.Diagnostics.Debug.WriteLine("c1");
	}
}
public class C2: C1
{
	public override void test()
	{
		System.Diagnostics.Debug.WriteLine("c2");
	}
}


Result:
new C1().test();
  -> c1
new C2().test();
  -> c2
I1 i=new C2(); i.test();
  -> c2