Speaking of patterns...

A lot of patterns do share a 'trick' that can be hard to grasp for new OO-developers. It's the trick which is made possible by polymorphism: you define a virtual method in a base class B, override it in a derived class D, and suddenly code in class B will call code in class D. To me this sounded confusing at first (hey, I'm raised with Kernighan-Ritchie C, the non-ANSI version, bite me :P). However it's a very neat way to 'plug in' code later (in a derived class) while defining the flow of the code now (in the abstract / base class).

A simple example for the few confused ones might help:

public class B
{
	public void Foo()
	{
		Bar();
	}

	public virtual void Bar()
	{
		Console.WriteLine("B.Bar");
	}
}

public class D:B
{
	public override void Bar()
	{
		Console.WriteLine("D.Bar");
	}
}