Virtual In C#
By Ravinder Bassi

Methods, properties, and indexers can be virtual, which means that their implementation can be overridden in derived classes.

The example:

using System;
class CAbc
{
 public virtual void F() { Console.WriteLine("CAbc.F"); }
}
class CBcd: CAbc
{
 public override void F() { 
  base.F();
  Console.WriteLine("CBcd.F"); 
 }
}
class Test
{
 static void Main() {
  CBcd b = new CBcd();
  b.F();
  CAbc a = b; 
  a.F();
 }
}