Figure 1-21. Passing a copy of the object to the server and getting a copy back in Visual Basic .NET

Printing PDF417 in Visual Basic .NET Figure 1-21. Passing a copy of the object to the server and getting a copy back

Figure 1-21. Passing a copy of the object to the server and getting a copy back
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This is potentially very dangerous, since other references to the original object continue to point to that original object only this one particular reference is updated. You can potentially end up with two different versions of the same object on the machine, with some references pointing to the new one and some to the old one.
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s Note
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If you pass a mobile object by reference, you must always make sure to update all references to use the new version of the object when the method call is complete.
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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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You can choose to pass a mobile object by value, in which case it s passed one way: from the caller to the method. Or you can choose to pass an mobile object by reference, in which case it s passed two ways: from the caller to the method and from the method back to the caller. If you want to get back any changes the method makes to the object, use by reference. If you don t care about or don t want any changes made to the object by the method, use by value. Note that passing a mobile object by reference has performance implications it requires that the object be passed back across the network to the calling machine, so it s slower than passing by value.
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Complete Encapsulation
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Hopefully, at this point, your imagination is engaged by the potential of mobile objects. The flexibility of being able to choose between local, anchored, and mobile objects is very powerful, and opens up new architectural approaches that were difficult to implement using older technologies such as COM. I ve already discussed the idea of sharing the Business Logic layer across machines, and it s probably obvious that the concept of mobile objects is exactly what s needed to implement such a shared layer. But what does this all mean for the design of the layers In particular, given a set of mobile objects in the business layer, what s the impact on the UI and Data Access layers with which the objects interact
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Impact on the UI Layer
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What it means for the UI layer is simply that the business objects will contain all the business logic. The UI developer can code each form or page using the business objects, thereby relying on them to perform any validation or manipulation of the data. This means that the UI code can focus entirely on displaying the data, interacting with the user, and providing a rich, interactive experience. More importantly, because the business objects are mobile , they ll end up running in the same process as the UI code. Any property or method calls from the UI code to the business object will occur locally without network latency, marshaling, or any other performance overhead.
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Impact on the Data Access Layer
A traditional Data Access layer consists of a set of methods or services that interact with the database, and with the objects that encapsulate data. The data access code itself is typically outside the objects, rather than being encapsulated within the objects. This, however, breaks encapsulation, since it means that the objects data must be externalized to be handled by the data access code. The framework created in this book allows for the data access code to be encapsulated within the business objects, or externalized into a separate set of objects. As you ll see in 7, there are both performance and maintainability benefits to including the data access code directly inside each business object. However, there are security and manageability benefits to having the code external. Either way, the concept of a Data Access layer is of key importance. Maintaining a strong logical separation between the data access code and business logic is highly beneficial, as discussed earlier in this chapter. Obviously, having a totally separate set of data access objects is one way to clearly implement a Data Access layer. However, logical separation doesn t require putting the logic in separate classes. It is enough to put the data access code in clearly defined data access methods. As long as no data access code exists outside those methods, separation is maintained.
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