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When fully implemented, the IQueryable interface is the mechanism by which anyone can provide his own implementation of LINQ. LINQ to XML, LINQ to SQL, and dozens of other LINQ providers all implement this interface. Thus, in order to provide the expected behavior when you run a LINQ query against BusinessListBase, IQueryable has been implemented for BusinessListBase. The IQueryable interface mandates only three members be implemented. Table 14-2 shows which members are required for an IQueryable implementation.

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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with the diagnostic engine, use the management API to control the infrastructure, and provide some code to control how all that works. You want to respond to events and keep your system healthy so you don t come in on Monday to find that you have 1,500 instances running in the cloud. In keeping with our heating and cooling metaphor, let s start with the thermostat.

Provides the source type for the implementation The most recent expression tree associated with the collection The IQueryProvider implementation used by IQueryable

As discussed in s 2 and 4, the data portal supports three transactional models: manual, Enterprise Services, and System.Transactions. The preferred model for performance and simplicity is System.Transactions, and so that is the model used in the sample application. This means that each method that updates data will be decorated with the [Transactional (TransactionTypes.TransactionScope)] attribute. Since this tells the data portal to wrap the code in a TransactionScope object, there s no need to write any ADO.NET or stored procedure transactional code. All the transaction details are handled by the TransactionScope object from System. Transactions. As you look at the data access code, notice that it never actually catches any exceptions. The code leverages using blocks to ensure that database connection, command, and data reader objects are disposed properly, but no exceptions are caught. The reasons for this are twofold: First, the code uses the [Transactional()] attribute, which causes it to run within a System. Transactions transactional context. An exception automatically causes the transaction to be rolled back, which is exactly the desired result. If the exceptions were caught, then the transaction wouldn t be rolled back, and the application would misbehave. Second, if an exception occurs, normal processing shouldn t continue. Instead, the client code needs to be notified that the operation failed, and why. Returning the exception to the client code allows the client code to know that there was a problem during data access. The client code can then choose how to handle the fact that the object couldn t be created, retrieved, updated, or deleted. Remember that the original exception is wrapped in a DataPortalException, which includes extra information that can be used by the client when handling the exception.

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