.NET Best Practices: Architecture & Design Patterns

Principles to get acquainted with

SOLID principles

When software development first came to life. People that wanted to start developing, obviously had no prior knowledge. Thus, everyone started applying their own ways of how they thought software should be written. The 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

More loose coupling using dependency injection

When writing code, you have to make sure you are not introducing unnecessary dependencies on other objects. Dependencies will just cost you on the long run since they are harder to maintain, not easily testable, exchangeable, ... . If you need to depend on another, then depend on the interface. Interfaces describe the dependency, but do not force it. Now, you don't want those dependencies to be filled hard coded and this is where dependency injection (DI) can help you. Dependency injection can help you at runtime dynamically deciding which implementations to plug into the dependencies.

• What is tight coupling and how to prevent it?
• Using Inversion of Control (IoC) containers.
• Constructor and Property injection.
• Microsoft.Extensions.DependencyInjection as an example.

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.

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 the .NET runtime can help at implementing it
• Builder - 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

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 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, ...
• Strategy - template method without the annoying inheritance
• Chain of Responsibility - strategy to go through a chain of strategies
• State - defer state dependending logic to state classes, state machines, 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, event vs. delegate
• Mediator - providing two-way communication between objects unaware of one another, correcly implementing INotifyPropertyChanged
• Visitor

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
• Decorator - altering the behavior of an object without the caller realising it
• 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.
• Building your own interpreter for simple math.
• 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.
• 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.

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.

Unit Testing

Unit testing in .NET

Every developer's dream: maintainable code that works. How do you keep your code maintainable? And how do you ensure some small change does not break your code? Unit testing to the rescue!

• What every developer wants: Quality code that works; and keeps on working.
• Finding bugs: not just in code. - Or how NASA lost a 125.000.000$ Mars orbiter.
• What is unit testing? And what is a good unit test?
• Understanding the difference between a unit test and an integration test.
• Test Driven Development - Should you be doing it?
• Role-playing: Marge, Homer and Bart
• The Triple-A of unit testing - and what has cooking to do with this?

Unit testing with Visual Studio

In this chapter we will look at building unit tests with the built-in features of Visual Studio.

• MSUnit - Built into Visual Studio
• Building and running Unit Tests with MSUnit.
• Using the Test Explorer Window

Architectural Patterns

Domain Driven Design

Getting the software right is a hard thing to do. This is because business and developers do not always talk the same language. Domain Driven Design is all about getting the language right between them - the ubiquitous language. We'll discuss the patterns used in DDD, and also look at some anti-patterns.

• What is Domain Driven Design?
• The Ubiquitous Language.
• Anti-Pattern: Bug Ball of Mud.
• Adding Bounded Contexts.
• Entities vs. Value Objects.
• Anti-Pattern: Death by a thousand dots.
• Using aggregate roots.
• Building Domain Services.
• Repository: keep the database out of your code.

Architecting Modern Web Applications

So what is a modern web application? One that uses Angular and TypeScript? With the speed that modern web application developement evolves, there is the chance that your web site uses 'old' technology by the time it is done. But sound architecture never goes stale. As a wise man once said: 'If you think good architecture is expensive, try bad architecure'.

• S.O.L.I.D. principles.
• Separation of Concerns, Dependency Inversion, Explicit Dependencies, Persistence Ignorance.
• "Clean" architecture: put the business and application model at the center.
• Easily tested with Automated tests

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.