authorize.net error code 128 A sample domain model in VS .NET

Maker Code 128 Code Set B in VS .NET A sample domain model

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Domain Driven Design Quickly by Abel Avram Floyd Marinescu. A 104-page book designed to be a more concise guide to DDD than Evans book. This ebook is summarized mainly from Evans book. (Lulu Press, 2007.) Applying Domain-Driven Design and Patterns: With Examples in C# and .NET by Jimmy Nilsson. The author takes the reader through real, complete examples and applies DDD patterns along with test-driven development (TDD) and O/R mapping. (Addison-Wesley Professional, 2006.) Domain-Driven Design Community (http://domaindrivendesign.org/). An evolving information website maintained by Eric Evans, Jimmy Nilsson, and Ying Hu.

We included a sample domain model in the example code for this book. In figure 8.1, you can see this sample domain model, and we ll work with different pieces of it in the rest of this chapter.

Figure 8.1 shows some of the entities and value objects in play within our domain model. The entities are the important objects in our domain model, such as Customer, Order, Product, and Supplier. With so many types in the diagram, you re probably wondering what s special about these classes and what makes them entities. The defining characteristic of an entity is that it has the concept of an identity, a property that can be examined to determine uniqueness. The reason we give these objects an identifier is that they can stand on their own, and we can speak about these objects without other supporting concepts. It would make sense to list a collection of any of these objects. Entities can stand on their own, and we can think about them in a collection or as a single object. Value objects don t make sense on their own without the supporting context of an entity to which they belong. Some value objects in our domain model are CustomerPriority and Address. Also, many properties of entities are value objects. Let s discuss CustomerPriority and what context is required for it to make any sense. A CustomerPriority has a value that indicates the priority level of the customer. It belongs completely to the Customer class; without Customer, CustomerPriority would have no context and would have no meaning. As a value object, CustomerPriority is defined by its properties and methods and has no identifier. It wouldn t make sense to list a collection or array of CustomerPriority instances because, without the Customer, it has no meaning or purpose. Its relationship with other entities gives it meaning. The Customer it belongs to and the status information it includes give it the context to convey meaning in the application, and when some other code needs the customer s priority, it must ask the Customer instance for the CustomerPriority. The Customer object will hand back this object. Like CustomerPriority, other types without identifiers are value objects. Value objects aren t glamorous, and even describing them can be boring. The arrangement of entities and value objects into larger structures can be interesting. Entities and value objects are useful in separating responsibilities in a domain model, but there s more. If we need to load a Product entity, what does that mean We see that our Product object can have many ProductCategory(s), and that each ProductCategory has a parent ProductCategory. Going further, a Product has a Price property. Orders and Suppliers all have a relationship with a Product. When we need to deal with a Product object, must we have all associated objects in memory for any operation to make sense The answer is no. In DDD, we divide our domain model into what are called aggregates.

Aggregates are groups of objects that work and live together. We group them along natural operational lines, and one entity serves as the aggregate root. The aggregate root is the entry point and the hub of operations for all objects in the aggregate. An aggregate can have many objects, or it can just be a single entity, but the aggregate