zxing pdf417 c# Figure 3-6. The ResourceAssignments collection and the ResourceAssignment child object in C#.NET

Encoder PDF-417 2d barcode in C#.NET Figure 3-6. The ResourceAssignments collection and the ResourceAssignment child object

Figure 3-6. The ResourceAssignments collection and the ResourceAssignment child object
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The AssignTo() method accepts a projectId parameter to identify the project to which the resource should be assigned.
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Can the Classes Be Merged
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It is important to notice that the objects described by Figure 3-5 and Figure 3-6 are similar, but they are not the same. Yet they do share at least some common information, if not behavior. Both child classes contain Assigned and Role properties, implying that there s commonality between them. Such commonality is not justification for combining the two classes into one, because their behaviors are distinctly different. The items in ProjectResources have one responsibility: managing information about a resource assigned to a project. The items in ResourceAssignments have a different responsibility: managing information about a project to which a resource is assigned. While this difference may seem subtle, it is a difference nonetheless. It is tempting to consider that the two classes could be merged into one, as shown in Figure 3-7. Of course, ProjectName isn t valid if the user got to this object from a Project object, but it is valid if she got here through a Resource object. The same is true for several other properties.
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Figure 3-7. Merged child items with assignment information Perhaps business logic could be added to properties to throw exceptions if they were called from an inappropriate context. But the obvious complexity of this sort of logic should give you pause. The problem is that one object is trying to handle more than one responsibility. Such a scenario means that the object model is flawed. Going down such a path will lead to complex, hard-to-maintain code.
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Historically, this sort of complex code was referred to as spaghetti code. It turns out that with improper object design, it is very possible to end up with spaghetti code in business objects. The result is terrible, and is exactly what good object design is intended to prevent.
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It should be quite clear at this point that merging the two collections or their child objects into a single set of objects isn t the right answer. They have different responsibilities, and so they should be separate objects. But this leaves one glaring issue: what about the common properties and any common business logic they might require How can two objects use the same data without causing duplication of business logic
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Dealing with Common Behaviors and Information
When designing relational databases, it is important to normalize the data. There are many aspects to normalization, but one of the most basic and critical is avoiding redundant data. A given data element should exist exactly once in the data model. And that s great for relational modeling. Unfortunately, many people struggle with object design because they try to apply relational thinking to objects. But object design is not the same as relational design. Where the goal with relational design is to avoid duplication of data, the goal of object design is quite different. There s no problem with a data field being used or exposed by different objects. I realize this may be hard to accept. We ve all spent so many years being trained to think relationally that it is often hard to break away and think in terms of objects. Yet creating a good object model requires changing this mode of thought.
Caution
Object design isn t about normalizing data. It is about normalizing behavior.
The goal in object design is to ensure that a given behavior exists only once within the object model. Simple examples of behavior include the idea of a string being required, or one value being larger than another. More complex behaviors might be the calculation of a tax or discount amount. Each behavior should exist only once in the object model, though it may be used from many different objects. This is why collaboration is so critical to good object design. For example, one object the DiscountCalculator will implement the complex calculation for a discount. Many other objects may need to determine the discount, so they collaborate with DiscountCalculator to find that value. In this manner, the behavior exists exactly once in the model.
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