One of the roles of the software architect is to act as a mentor/coach. Reviewing some of the designs in one of my projects' teams it seems the time was ripe for doing just that. Thus, last week I gave them a presentation on the basics of good OO design  - which I thought might also be of interest for other people (you can download a copy here - 312KB).

The presentation starts with the  7 deadly sins of software design:

• Rigidity – make it hard to change
• Fragility – make it easy to break
• Immobility – make it hard to reuse
• Viscosity – make it hard to do the right thing
• Needless Complexity – over design
• Needless Repetition – error prone
• Not doing any

It is interesting to note that just yesterday I read an interesting piece on what makes good design (i.e. looking from the positive side) by James Shore (found via Sam Gentile) 

The main part of the presentation demonstrates the 5 basic design principles (drafted by people like Robert C. Martin , and Barbara Liskov ):

• OCP open-closed principle - a class should be open for extension but closed for modifications
• SRP single responsibility principle - a class should have a single responsibility
• ISP interface segregation principle - there should be separate interfaces for different consumer types
• LSP Liskov substitution principle - basically design by contract - a sub-class should fulfill the same expectations its suparclass set
• DIP dependency inversion principle - classes should depends on abstractions, class consumers should depend on abstractions and abstractions shouldn't depend on details.

These principles are the basis for  some of the techniques widely used today - few examples include:

Inversion Of Control - builds on OCP

Dependency Injection - a mechanism to allow DIP

Contract First - building on LSP,DIP

At the end of the day following these principles helps managing classes dependencies, increase overall loose coupling and cohesion thus increasing the overall quality of design. It sometimes amazes me how using just a  few simple rules can improve maintainability, flexibility and usefulness of designs so much.