.NET Best Practices: Architecture & Design Patterns

Principles to get acquainted with

SOLID principles

SOLID principles were introduced by Robert C. Martin (a.k.a Uncle Bob) in 2000. The intention of these principles is to make software designs more understandable, easier to maintain and extend. These principles are essential for every developer to know, because it will help them in writing better code an better understanding other code that was written with these principles in mind.

• S - Single Responsibility
• O - Open/Closed
• L - Liskov Substitution
• I - Interface Segregation
• D - Dependency Inversion

Design Patterns

Introduction to Patterns

One might not believe it, but design patterns do not find their origin in software architecture. Patterns were first described in the 1970s by a real "Bricks and Stones" architect. A pattern is defined as a re-usable solution to a common type of problem, without ever repeating the actual way the solution is implemented. This means that patterns are an ideal learning tool and also ideal for communicating design ideas. Of course there are also anti-patterns, things often used, but actually better avoided.

• What is a Pattern?
• The Gang of Four: Erich Gamma, Ralph Johnson, Richard Helm and John Vlissides.
• Different kinds of design patterns: creational, structural and behavioral patterns.
• Patterns everywhere: the difference between implementation, design and architecture patterns...
• When to apply patterns, and when not to.
• Some anti-patterns such as Loosy-Goosy.

Creational patterns

When creating software, you feel yourself continuously creating new objects. Although the basic form of creating new objects is not bad, it can result in design problems or added complexity to the design. Creational patterns will help you deal with object creation that is suitable to the situation.

• Singleton - a.k.a. The Pluto Pattern and how to implement it in .NET
• LAB: Building a Singleton in .NET
• Builders - seperating the construction from the representation
• Factory Method - delegating/hiding the creation of objects to a factory
• Abstract Factory - abstracting the factory to create families of objects
• LAB: Implementing an Abstract Factory

Behavioral patterns

Once your objects are created, they start interacting. When you're not careful, these interactions can quickly start leading into code that is tightly coupled. Behavioral design patterns help with identifying common communication patterns between objects and realize these patterns. By correctly applying these patterns, you can increase flexibility in carrying out the interaction between objects.

• Template Method - defer exact parts of an algorithm to inheriting classes, delegates, ...
• LAB: Building a Template Method for a board game
• Strategy - template method without the annoying inheritance
• LAB: Implementing a Builder as a Strategy
• Chain of Responsibility - strategy to go through a chain of strategies
• LAB: Using a Chain of Responsibility to implement a Builder
• State - defer state dependending logic to state classes, state machines,
• Using the Stateless library
• LAB: Implementing a VCR with the State patterns using the Stateless library
• Iterator - providing a generic way of navigating through collections, yield is your friend, asynchronously iterating a collection
• Observer - notifying whoever is interested in what you have to say, events vs. delegates
• Mediator - providing two-way communication between objects unaware of one another, correcly implementing INotifyPropertyChanged
• LAB: Avoiding the String-based programming anti-pattern

Structural patterns

Created objects do not tend to stand on their own. No, they start encapsulating other objects trying to create structure in, what otherwise would be a chaotic software environment. Structural patterns help at identifying and setting up relationships between objects.

• Adapter - plugging in different objects into your code that do not fit
• LAB: Building INotifyPropertyChanged as a generic Adapter
• Decorator - altering the behavior of an object without the caller realising it
• LAB: Changing IComparable<T> with a Decorator
• Composite - tree structures are here to help you, working with Linq Expressions
• Facade - hiding the complexity of subsystems from the caller
• Flyweight - reduce memory consumption by preventing unnecessary creation of object
• Proxy - proxying requests made to the subject without changing the behavior

Building your own little programming language with some patterns - Fun!

Programming languages also use a bunch of patterns in their implementation. In this chapter you will build your own little programming language, which is extensible by the way. While doing so you will discover and apply some very important patterns which are perfectly useable outside of the scope of building programming languages!

• Interpreter: Build your own expressive language-grammar and execute it.
• How LINQ uses Interpreter - and how you can take advantage of it yourself.
• Builder: Hide how complex hierarchies of objects get built - and allow variations.
• XAML as the ultimate builder.
• Reflection: the ideal .NET way for implementing your own builder.
• How NOT to use reflection.
• LAB: Building your own calculator with Interpreter and Builder
• Visitor: When you need a lot of different operations on the same object structure.
• Building a pretty-printer using Visitor.
• Implementing Visitor the dynamic way.
• LAB: Implementing a Visitor to walk over a complex hierarchy of objects
• Automating tedious code with Source Generators

Model-View-Whatever

Most developers make lousy graphic designers. That is why we see the emergence of patterns that allow developers to focus on writing code to implement the behavior of the application, while allowing graphics designers to build kick-ass user interfaces. The main pattern is called Model-View-*Whatever*, with *Whatever* replaced depending on the technology you are using. Understanding the MVW pattern is important as it is used to develop both windows and web applications.

• Model-View-Controller: An ancient pattern back in fashion.
• ASP.NET MVC - an introduction.
• MVVM in WPF - MVC taking advantage of powerful databinding capabilities.
• Command: Encapsulate behavior in objects.
• Implementing commands using closures.
• LAB: Using MVVM in a WPF application

Reactive Programming

Have you ever heard of Reactive Programming? This is an important new development where you learn to program using Observables. You will learn to apply this technique to solve some common, hard-to-solve problems with classic Object-Oriented Programming, and we will also explore the Fluxor/Redux pattern.

• What are Observables?
• Using Reactive Extensions.
• Applying the Redux pattern so simplify complex applications.
• LAB: Implementing a search system with Observables

Framework building

Design Patterns Applied: Developing your own reusable library.

When do you need patterns the most?! When you are building a framework yourself. Building a framework means handling new features while keeping backward compatibility. This very hard task can be make easier with the proper use of patterns. So in this last part of the training we will build a reusable library, and while doing so encounter some problems and then solve these problems by applying the right pattern.

• Adding the GoF Command pattern to MVVM.
• Using interfaces for flexibility.
• Building Command Objects - extending WPF's ICommand interface.
• Adding Undo and Redo functionality to the command pattern.
• Using a CommandManager class.
• Challenge: retro-fitting our commands into MVVM without lots of changes.
• Implementing Undo-Redo using the Memento pattern.
• Choosing whether or not to add the Prototype pattern.
• Ideas on how to proceed with the command pattern.
• LAB: Implementing Undo/Redo logic as a reusable library