UML and some stuff

Here are some useful links for UML class diagrams
http://www.objectmentor.com/resources/articles/umlClassDiagrams.pdf
http://www.ibm.com/developerworks/rational/library/content/RationalEdge/sep04/bell/

And a book, thanks to Asmaa Magdi ACMaa
http://portal.aauj.edu/portal_resources/downloads/uml/sybex_mastering_uml_with_rational_rose2002.pdf

The difference between Aggregation and Association

Aggregation is when an object contains a collection of other object. This is done mainly via data structures. Like a class that contains an Array, a List, a Dictionary, a HashTable, ...

Association is when objects/classes (sub-systems) interact together in a larger system. This can be achieved either by letting the objects have references to other objects in the system (this involves aggregation), by using static classes, ...

Examples,

1. In Asp.Net, the use of static classes is obvious. You have the MembershipProvider, the RuleProvider, the ConfigurationManager, ... which provide services (functionality) to other objects/classes.
2. Suppose you're designing a strategy game where soldier must know the location of other soldiers. So you will make a list of references to other soldiers in the soldier class.

Object Composition at wikipedia

This file contains a sample C# code and it's simple UML class diagram (in both Word 97-2003 and Word 2007 formats)