////   This file contains the C# code from Program 5.18 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/pgm05_18.txt//public class Association : ComparableObject{    protected IComparable key;    protected object value;    public override int CompareTo(object obj)    {	Association association = obj as Association;	return key.CompareTo(association.key);    }    public override string ToString()    {	string result = "Association {" + key;	if (value != null)	    result += ", " + value;	return result + "}";    }    // ...}