////   This file contains the C# code from Program 4.16 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C#"//   by Bruno R. Preiss.////   Copyright (c) 2001--2002 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.brpreiss.com/books/opus6/programs/pgm04_16.txt//public class LinkedList{    protected Element head;    protected Element tail;    public object First    {	get	{	    if (head == null)		throw new ContainerEmptyException();	    return head.datum;	}    }        public object Last    {	get	{	    if (tail == null)		throw new ContainerEmptyException();	    return tail.datum;	}    }    // ...}