OO and Design Principles to strive for

Introduction

Often, while developing software, we can get carried away and "hack" away until we get it working. However, it's sometimes handy to have a guideline.

So here's my pin up list of design principles to strive toward. These are only guidelines, and should be treated as such. They are not rules to follow...

• Hide details of the implementation: Code to contracts and not implementations. Contracts are either abstract classes or pure interfaces.
• Keep coupling to a minimum: A class can depend on native types (basic ints, bools, etc.) but should only depend on a select few peer classes. If the dependencies are high, consider this a smell and refactor to reduce the dependency. This can include creating new specialized types that are composed of the dependencies, but again this must be managed correctly.
• Maintain maximum cohesion: A class should focus on a single concept.
• Favour composition over inheritance: There’s a few different ways to get objects to collaborate – inheritance, composition, subscription through event handling.
• Adhere to the Open closed principle: An object should be open for extension (able to extend its behaviour), but closed for modification (while extending the behaviour, you should not modify its source code)
• Follow LSP where possible. Liskov Substitution principle: objects that have references to base types should be able to use derived types without knowing the difference. The basic concept behind polymorphism. TIP: design objects around behaviour and not data.
• Use Inversion of control / Dependency Inversion principle: A formal definition: High level modules should not depend upon low level modules. Both should depend upon abstractions. Abstractions should not depend upon details. Details should depend upon abstractions.
• Employ the Interface Segregation principle: Objects should not be forced to depend on interfaces that they don't use.
• Consider using the Law of Demeter: Only talk to your immediate friends, so the following would be considered a violation:
  • C#

    var payment= Client.Wallet.GetMoney(10.00); 

  • This will be an interesting one to enforce if you plan on sticking to Domain driven design principles which suggest that you should get to entities through their aggregates.
• Encapsulate the variations: When encountering variants, encapsulate those bits that vary and make them first class citizens.
• Consider using Factories for object creation.

The above are principles and guidelines and violating them is okay, but only if you understand why doing so is better than following them.

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