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calls (or other types of resource calls) on behalf of the repositories (Nilsson, Applying Domain - Driven. Design and Patterns, With Examples in C# pixia-club.info Primarily a developer, but also a trainer and author. • Blogs at pixia-club.info weblog/. • Author of ”Applying Domain-Driven. Design and Patterns” and ”.NET. Applying Domain-Driven Design and Patterns: With Examples in C# pixia-club.info Home · Applying NET Patterns: Architecture, Design, and Process · Read more .

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NET Domain-Driven Design with C# (eBook) available to buy online at takealot. com. Many ways to pay. Tim McCarthy; Adobe DRM PDF. Problem - Design -. NET Domain-Driven Design with C#: Problem - Design - Solution of building a real-world application using Domain-Driven Design implemented in C#. Domain-Driven. Design and Patterns. With Examples in C# pixia-club.info Jimmy Nilsson r\Addison-Wesley. T T. Upper Saddle River, NJ • Boston • Indianapolis.

The quality of the upstream model, along with the fit of the upstream model may be 'good enough'. That suggests you would not want a context where you were working on the core domain to be in a conformist relationship.

Anti-corruption Layer When contexts exist in different systems and attempts to establish a relationship result in the 'bleeding' of one model into the other model, then the intention of both will be lost in the mangled combination of the models from the two contexts.

In this case, it is better to keep the two contexts well apart and introduce an isolating layer in-between that is responsible for translating in both directions. This anti-corruption layer allows clients to work in terms of their own models. Anti-corruption Layer is a great pattern for dealing with legacy systems or with code bases that will be phased out Separate Ways Critically analyze the mappings between bounded contexts.

If there are no indispensable functional relationships, then keep the context separate. The rationale is that integration is costly and can yield very low returns. Section 5 Modeling the Domain Within the bounded contexts, effort is focused on building really expressive models; models that reveal the intention more than the implementation.

When this is achieved, concepts in the domain surface naturally and the models are flexible and are simpler to refactor. The DDD patterns are more of an application of patterns from GoF, Fowler and others specifically in the area of modeling subject domains.

The most common patterns are described below. Dealing with Structure Entities Entities are classes where the instances are globally identifiable and keep the same identity for life. There can be change of state in other properties, but the identity never changes. In this example, the Address can change many times but the identity of the Client never changes, no matter how many other properties change state.

Value Objects Value objects are lightweight, immutable objects that have no identity. While their values are more important, they are not simple data transfer objects.

Value objects are a good place to put complex calculations, offloading heavy computational logic from entities. They are much easier and safer to compose and by offloading heavy computational logic from the entities, they help entities focus on their role of life-cycle trackers.

In this example, when the address of the Client changes, then a new Address value object is instantiated and assigned to the Client. Value Objects have simple life cycles and can greatly simplify your model. They also are great for introducing type safety at compile time for statically typed languages, and since the methods on value objects should be side effect free, they add a bit of functional programming flavor too.

Cardinality of Associations The greater the cardinality of associations between classes, the more complex the structure. Aim for lower cardinality by adding qualifiers. Bi-directional associations also add complexity. Critically ask questions of the model to determine if it is absolutely essential to be able to navigate in both directions between two objects. In this example, if we rarely need to ask a Person object for all its projects, but we always ask a Project object for all people in the roles of the project, then we can make the associations one directional.

Direction is about honoring object associations in the model in memory. If we need to find all Project objects for a Person object, we can use a query in a Repository see below to find all Projects for the Person. Services Sometimes it is impossible to allocate behavior to any single class, be it an entity or value object. These are cases of pure functionality that act on multiple classes without one single class taking responsibility for the behavior.

In such cases, a stateless class, called a service class, is introduced to encapsulate this behavior. Aggregates As we add more to a model, the object graph can become quite large and complex. Large object graphs make technical implementations such as transaction boundaries, distribution and concurrency very difficult.

Aggregates are consistency boundaries such that the classes inside the boundary are 'disconnected' from the rest of the object graph.

Each aggregate has one entity which acts as the 'root' of the aggregate. When creating aggregates, ensure that the aggregate is still treated as a unit that is meaningful in the domain. Also, test the correctness of the aggregate boundary by applying the 'delete' test.

In the delete test, critically check which objects in the aggregate and outside the aggregate will also be deleted, if the root was deleted.

Follow these simple rules for aggregates. The root has global identity and the others have local identity The root checks that all invariants are satisfied Entities outside the aggregate only hold references to the root Deletes remove everything in the aggregate When an object changes, all invariants must be satisfied.

Remember that aggregates serve two purposes: domain simplification, and technical improvements. There can be inconsistencies between aggregates, but all aggregates are eventually consistent with each other. Dealing with Life Cycles Factories Factories manage the beginning of the life cycle of some aggregates. This is an application of the GoF factory or builder patterns. Care must be taken that the rules of the aggregate are honored, especially invariants within the aggregate.

Use factories pragmatically. Remember that factories are sometimes very useful, but not essential. Repositories While factories manage the start of the life cycle, repositories manage the middle and end of the life cycle. Repositories might delegate persistence responsibilities to object-relational mappers for retrieval of objects. Remember that repositories work with aggregates too. So the objects retrieved should honor the aggregate rules.

Dealing with Behavior Specification Pattern Use the specification pattern when there is a need to model rules, validation and selection criteria. The specification implementations test whether an object satisfies all the rules of the specification. If projectIsOverdueSpecification. In this pattern, the varying 'part' is factored out. Consider the following example, which determines the success of a project, based on two calculations: 1 a project is successful if it finishes on time, or 2 a project is successful if it does not exceed its budget.

The important point to remember is that the client code should only deal with the abstract type representing the composite element. Consider the following class. A Milestone is a Task with a due date but no duration. Applying a composite pattern, we can introduce a new type Activity with different implementations. Section 6 Application Architecture When the focus of design is on creating domain models that are rich in behavior, then the architecture in which the domain participates must contribute to keeping the model free of infrastructure too.

Typically, a layered architecture can be used to isolate the domain from other parts of the system. Each layer is aware of only those layers below it. As such, a layer at a lower level cannot make a call i. Also, each layer is very cohesive and classes that are located in a particular layer pay strict attention to honoring the purpose and responsibility of the layer. User Interface Responsible for constructing the user interface and managing the interaction with the domain model.

Typical implementation pattern is model-view-controller. Application Thin layer that allows the view to collaborate with the domain. Warning: it is an easy 'dumping ground' for displaced domain behavior and can be a magnet for 'transaction script' style code.

Domain An extremely behavior-rich and expressive model of the domain. Note that repositories and factories are part of the domain. However, the object-relational mapper to which the repositories might delegate are part of the infrastructure, below this layer.

Infrastructure Deals with technology specific decisions and focuses more on implementations and less on intentions. Note that domain instances can be created in this layer, but, typically, it is the repository that interacts with this layer, to obtain references to these objects. Aim to design your layers with interfaces and try to use these interfaces for 'communication' between layers.

Also, let the code using the domain layer control the transaction boundaries. Section 7 Recently Added Patterns Big Ball of Mud This is a strategic design pattern to deal with existing systems consisting of multiple conceptual models mixed together, and held together with haphazard, or accidental, dependent logic.

In such cases, draw a boundary around the mess and do not attempt to try sophisticated modeling within this context. Be wary of this context sprawling into other contexts. Domain events are not to be confused with system events that are part of the software itself. It may be the case that domain events have corresponding system events that are used to carry information about the event into the system, but a domain event is a fully-fledged part of the domain model.

Model these events as domain objects such that the state of entities can be deduced from sets of domain events. Event objects are normally immutable since they model something in the past. In general, these objects contain a timestamp, description of the event and, if needed, some identity for the domain event itself.

Although we have a plethora of tools and frameworks to make it easier, we still have to figure out how to use these tools well. There are lots of approaches you can take, but the trick is knowing which one to use in specific situationshardly ever does one approach work in all cases. People involved in this effort, such as me, try to find common approaches and describe how to do them well and when they are applicable.

The resulting work is pretty wide ranging, and that can lead to too much choice for the reader. I struggled to find much of anything, but one rare book that tackled the territory was Jimmy's earlier book.

I liked his informal writing style and eagerness to dig into concepts that many others skimmed over. So it's fitting that Jimmy decided to take many of the ideas from me and the others in the enterprise patterns community and show how you can apply them in writing. NET applications. The focus of this enterprise patterns community is documenting good designs, but another thread runs through us. So Jimmy also brought these ideas into this book. Many people think that pattern-people's focus on design and TDD's focus on evolution are at odds.

The huge overlap between pattern-people and TDDers shows this isn't true, and Jimmy has weaved both of these threads into this book. The result is a book about design in the. It's a book that shows you how to begin applying such things as TDD, object-relational mapping, and domain-driven design to.

NET projects. If you haven't yet come across these concepts, you'll find that this book is an introduction to techniques that many developers think are the key for future software development. If you are familiar with these ideas, the book will help you pass those ideas on to your colleagues. Many people feel the Microsoft community has not been as good as others in propagating good design advice for enterprise applications.

As the technology becomes more capable and sophisticated, it becomes more important to understand how to use it well. This book is a valuable step in advancing that understanding.

That is what reading this book is like. This book does not push a new grand scheme. It unaffectedly reports on one expert practitioner's use of and combination of the current practices he has been drawn to. Furthermore, all three of these are harder than they look at first.

They require extensive knowledge over a wide range. This book does spend some time advocating these approaches, but mostly it focuses on the details of how to make them work. Effective design is not just a bunch of techniques to be learned by rote; it is a way of thinking. As Jimmy dives into an example he gives us a little window into his mind.

He not only shows his solution and explains it, he lets us see how he got there. When I am designing something, dozens of considerations flit through my mind. If they are factors I've dealt with often, they pass so quickly I am barely conscious of them.

If they are in areas where I have less confidence, I dwell on them more. I presume this is typical of designers, but it is difficult to communicate to another person. As Jimmy walks through his examples, it is as if he were slowing this process down to an observable pace.

At every little juncture, three or four alternatives present themselves and get weighed and rejected in favor of the one he eventually chooses. For example, we want model objects that are implemented free of entanglement with the persistence technology.

Applying Domain-Driven Design and Patterns: With Examples in C# and .Net

So what are eight ways eight! What considerations would lead you to compromise on some of these points? What do the currently popular frameworks, including the. NET platform, impose? Jimmy thinks pragmatically. He draws on his experience to make a design choice that will likely take him toward the goal, adhering to the deeper design principle, rather than the choice that looks the most like a textbook example. And all of his decisions are provisional. The first design principle Jimmy holds in front of himself is the fundamental goal of DDD: a design that reflects deep understanding of the business problem at hand in a form that allows adaptation to new wrinkles.

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So why so much discussion of technical framework and architecture? It is a common misperception, perhaps a natural one, that such a priority on the domain demands less technical talent and skill.

Would that this were true.

It would not be quite so difficult to become a competent domain designer. Ironically, to render clear and useful domain concepts in software, to keep them from being suffocated under technical clutter, requires particularly deft use of technology.

My observation is that those with the greatest mastery of technology and architectural principles often know how to keep technology in its place and are among the most effective domain modelers. I do not refer to the knowledge of every quirk of complex tools, but to the mastery of the sort of knowledge laid out in Martin Fowler's PoEAA, because nave application of technology paradoxically makes that technology more intrusive into the application.

For many people this book will fill in gaps of how to implement expressive object models in practice. I picked up a number of useful ways of thinking through the application of technical frameworks, and I especially firmed up my understanding of some particulars of doing DDD in a. NET setting. In addition to technical architecture, Jimmy spends a great deal of time on how to write tests. In the absence of a focus on refining an ever more useful model, TDD is prone to fragmented applications, where a single-minded attack on one feature at a time leads to an unextendable system.

A comprehensive test suite actually allows such a team to continue making progress longer than would be possible without it, but this is just the basest value of TDD.

At its best, the test suite is the laboratory for the domain model and a technical expression of the ubiquitous language. Tests of a particular style drive the modeling process forward and keep it focused. This book steps us through examples of developing such tests.

Jimmy Nilsson has a rare combination of self-confidence and humility, which I have observed to be characteristic of the best designers. We get a glimpse of how he got to his current understanding as he tells us what he used to believe and why that opinion changed, which helps to take the reader past the specifics of the techniques to the underlying principles.

This humility makes him open to a wide range of influences, which gives us this fusion of ideas from different sources.

He has tried a lot of things and has let his results and experience be his guide. His conclusions are not presented as revealed truth, but as his best understanding so far with an implicit recognition that we never have complete knowledge. All this makes the advice more useful to the reader. And this attitude, in itself, illustrates an important element of successful software development leadership.

It seems that all software architecture books must have a bridge on the cover, but there are some additional reasons the bridge is appropriate for this book. This bridge replaced a ferry that I took many times as a child. I enjoy very much driving over it even after dozens of times. On a personal note, my father was on the team that built the highest parts of the bridge.

But beyond these, the main reason is that this book is very much about bridging gaps; bridging gaps between users and developers; bridging gaps between business and software; bridging gaps between logic and storage. Bridging gaps between "DB-guys" and "OO-guys" Hey, how geeky can a preface be?

Focus of This Book The main focus of the book is how a Domain Model could be constructed to be clean, yet still be persistence-friendly. It shows what the persistence solution could look like for such a Domain Model and especially how to bridge that gap between the Domain Model and the database.

Applying Domain-Driven Design and Patterns: With Examples in C# and .NET

DDD might be perceived as a bit abstract. Therefore, more concrete examples are helpful regarding persistence, for example. Mine may be fairly basic, but it is a platform to start from.

It has become very clear to me that "one size does not fit all" when it comes to architecture. Having said that, patterns have proven to be general enough to use and reuse in context after context.

The focus isn't on the patterns themselves, but this book uses patterns in every chapter as a tool and language for discussing different design aspects. A nice side effect is that patterns-ignorant readers will also gain some insight and interest into patterns along the way. That also goes for TDD. Not all developers have become interested in this yet.

I think it's especially common in the. NET community that TDD just as patterns is considered a niche technique at best, or it might even be totally unknown. Readers will learn how to apply TDD. Why This Book? Writing my first book [Nilsson NED] was a really tough project on top of all my other ordinary projects and obligations.

I was pretty sure I wouldn't write another, but the time came when I thought I had something to say that I couldn't leave unsaid.

My change of heart started when I read two recent books that inspired me and changed my thinking. This book inspired me to give the Domain Model pattern another try after having failed with several earlier attempts. This book provided me with insights about how to think and act regarding development with a strong domain focus and with a certain style of how to apply the Domain Model pattern. Another important influence was all that I learned from teaching my patterns course over a couple of years.

As I interacted with students and the material evolved, I had insights myself. My views of DDD transformed as I worked on an ambitious though unfortunately unfinished open source project called Valhalla, which I developed in collaboration with Christoffer Skjoldborg.

Christoffer did by far the most work. To summarize all this, I felt that a book that dealt more with application than theory was needed, but one that was based on solid ground, such as the DDD and PoEAA books. Target Audience This book is aimed at a wide target audience. It will help if you have some knowledge of Object-orientation. NET or a similar platform C or a similar language Relational databases; for example, SQL Server However, interest and enthusiasm will compensate for any lack of prior experience.

I'd like to elaborate on my statement that the target audience is wide. First, we can think about the way we put people into platform boxes.

The book should serve. NET people who want a more corebased approach than drag-till-you-drop if I may use some weak generalizations. NET people and Java people. Let's try to describe the target audience by using another dimension. Then I think that the book is for developers, team leaders, and architects. Choosing yet another dimension, I think there might be something in this book both for intermediate and advanced readers.

There's probably also something for beginners. We also introduce patterns and TDD. The chapters include the following: Chapter 1, "Values to Value" This chapter discusses properties of architecture and process to value for creating quality results when it comes to system development. The discussion is influenced by Extreme Programming. Chapter 2, "A Head Start on Patterns" This chapter focuses on providing examples and discussions about patterns from different families, such as design patterns, architectural patterns and domain patterns.

We also prepare the Domain Model for the infrastructure, and focus quite a lot on rules aspects. Chapter 4, "A New Default Architecture" This chapter lists a set of requirements of an example application, and a first-try model is created as a start for the coming chapters. A Domain Model-based architecture is used. Chapter 6, "Preparing for Infrastructure" Even though we try to push the infrastructure aspects as far off in the future as possible, it's good to think a little bit ahead and prepare the Domain Model for the infrastructure needs.

In this chapter, there is a lot of discussion about pros and cons of Persistence Ignorant Domain Models. Chapter 7, "Let the Rules Rule" This chapter talks about business rules in the form of validation and how a Domain Modelbased solution can deal with the need for such rules, connecting back to the requirements set up in Chapter 4.

We will take a look at how those requirements are fulfilled by an example tool.

Chapter 8, "Infrastructure for Persistence" When we have a fairly good Domain Model, it's time to think about infrastructure, and the main type of infrastructure in this book is infrastructure for persistence. This chapter discusses different properties of persistence solutions and how to categorize a certain solution.

Chapter 9, "Putting NHibernate into Action" This chapter uses the categorizations of the prior chapter with an example of a persistence solution, namely NHibernate [NHibernate]. Part IV: What's Next? In this part, there is a focus on other design techniques to keep an eye on and start using. The other focus is on how you can deal with the presentation layer when it comes to bridging that gap to the Domain Model, but also how to deal with developer testing of the UI.

This part is almost exclusively written by guest authors. Chapter 11, "Focus on the UI" This chapter focuses on how the UI can be connected to the Domain Model and how to increase testability for the user interface when using a Domain Model, both for rich client applications and Web applications. Appendices There are two appendices providing further examples of Domain Model styles and an overview-type patterns catalog. Why C for the Examples?

In no way is this a book for teaching C. But I still need a language or possibly several, but I have chosen one for the examples, and that's where C comes in. The reasons for the choice are mainly that C is my current main language and that most VB. NET and Java developers can read C code pretty easily. Regarding the version, most of the code examples work in both C 1. Topics That Aren't Covered There are loads of topics that aren't covered in the book, but I think there are two missing topics that spring to mind.

They are distribution and advanced modeling. Distribution It was in my early plans of the book to include thorough coverage of the distribution aspects, but later on I came to the conclusion that the book would become too unfocused. Still, there is some coverage here and there. Advanced Modeling The title of the book might suggest that you find advanced and interesting examples of modeling of certain problem areas. That's not exactly the case. On the contrary, it's a joint effort.

See the list of people in the Acknowledgments section, and also remember that even more people have been involved, especially during production.

That said, any errors we didn't catch before the book went to print are mine and mine alone. I will post information of interest to readers of the book at www.

Getting back to bridging gaps, the photo of the resund Bridge was taken by my friend Magnus von Schenck on one of his sailing trips. Even though this book has not been as tough to write as the first one, there has been a fair amount of blood, sweat, and tears. I hope the book might save you some of that. Have fun and good luck! Jimmy Nilsson www. My artist of the figures for the parts introductions: Kjell Warnquist My language editor: Lydia West My development editor: Chris Zahn My project editor: Michael Thurston My copy editor: Elise Walter My marketing manager: Curt Johnson My acquisitions editor: Karen Gettman and Sondra Scott who talked me into the project in the first place Without you there would be no book, or at least a book of inferior quality.

Thanks guys, I owe you! Part I: Background In the "Background" part, we discuss architecture and processes in general terms. The first part is about setting the scene. Romeo and Juliet must have a scene. I will do that by looking back in time over the last few years regarding how I have thought about different concepts and how my ideas have changed over time.

We will jump around quite a lot and cover a lot of ground, but the overall idea is to discuss values to value regarding architecture and processes for development. On this journey, we will introduce and talk about many concepts that we will discuss in depth later on in the book. Start the camera Overall Values In the past, I was very good at planning ahead.

I often added functionality, structures, and mechanisms to my projects proactively. That part usually turned out pretty well, but I often forgot about the artifacts added on that never came to any good use. Of course, I added loads of them, too. The cost was pretty large, both in development time and in increased complexity. Over the last few years, we've been encouraged to use another approach: "Do the simplest thing that could possibly work.

Domain-Driven Design

Both approaches are kind of the two extremes add all you can think of up front versus do the simplest thing , but I think they both miss something in that they don't address the tradeoffs they make. Just about every question regarding "is this or that best" can be answered with "it depends. I tend to prefer an approach that is somewhere in the middle, moving in one or the other direction depending upon the situation.

The word "lagom" is a Swedish word meaning something like "just right" or "not too much, not too little. Let's have a closer look at a couple of more specific areas of values architecture and process ingredients , starting with some aimed at architecture to get us into the right mood. Architecture Styles to Value Architects and developers must make design decisions based on the requirements of the project.

I haven't collected an exhaustive list of values regarding architecture styles here, but I have decided on a few main things I'd like to comment on to some extent, such as model focus, domain models, databases, distribution, and messaging. First, I think it's wise to keep a model focus. Focus on the Model For a long time, I have liked the object-oriented paradigm a lot, but it wasn't until pretty recently that I made the full move to that paradigm myself.

There have been platform problems with using the paradigm before, but now the platforms are mature. Note As a reviewer pointed out, maturity depends on the platform we are talking about.

If you come from a VB background, what was just said is reasonable, but if you come from Java, SmallTalk, C , and so on, the platform has been mature for quite some time.

We were discussing multiplicity between classes, where behavior should belong, and so on. I realized after a few sessions that using my technique for discussion with expert users, instead of their technique, was a complete failure. They answered my questions randomly and didn't see or add much value at all in the discussion. The system at large wasn't a failure, it's still being used and it's the core system there, but the development process didn't go as smoothly as it could have.

I had to change method. Use Case Focus I have thought about that experience several times and have actually joked about how nave I was. In the years following, I always tried to play by the methodologies of the target groups; I mean the methodologies of the users with the users, and the methodologies of software development with developers. At least it worked out well in the informal manner of XP [Beck XP] stories, short text descriptions, which was what I used.

Such a description is a short description of a piece of functionality in a system. An example is "Register Order for Company Customer. The way I did the bridging was to have one class per use case in the software. It did come to my attention that thanks to my way of applying use cases, I became pretty procedural in my thinking.

I was designing a little bit like Transaction Script [Fowler PoEAA], but I tried to balance it by generalizing as much behavior as possible or at least suitable. A few years ago I heard Ivar Jacobson talking about use cases, and I was pretty surprised when I realized that he didn't encapsulate the use cases in classes of their own as I had expected and had done for a long time.

Another thing that got me kind of worried about this method was the constant struggle I was having with my friend and Valhalla-developer Christoffer Skjoldborg when we worked with the Domain Model pattern [Fowler PoEAA]. He saw little value in the use case classes, maintaining that they could even get in the way because they might become a hindrance when mixing and matching use case parts.

Transaction Script is similar to batch programs in that all functionality is described from start till end in a method. Transaction Script is very simple and useful for simple problems, but breaks down when used for dealing with high complexity.

Duplication will be hard to avoid. Still, very many very large systems are built this way. Been there, done that, and I've seen the evidence of duplication even though we tried hard to reduce it. It crops up. To get the name, you call a method and send id 42 as a parameter. The Table Module uses a Recordset internally for answering the request. This certainly has its strengths, especially in environments when you have decent implementations for the Recordset pattern.I tried to use a visual model to communicate my understanding of the requirements of a system I was about to build.

These will give you a great foundation when you are working with your own Domain Model-based system. Third, building systems is something many more people have experience in and have been taught about now.

It should be time to increase the abstraction level. This bridge replaced a ferry that I took many times as a child.

HARRY from Deltona
Browse my other posts. I am highly influenced by spearfishing. I do fancy sharing PDF docs yearningly.