Virtual Classes

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();
}
}
This shows a class CAbc with a virtual method F, and a class CBcd that overrides F. The overriding method in B contains a call base.F() which calls the overridden method in A.