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Putting the VS 2005-Generated Data Access Layer Into Perspective : Page 4

Get an insider view of VS 2005 auto-generated Data Access Layers (DALs) so you can edit and extend the code.


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Extending a Table Adapter
When you design a business and data access layer, you basically map entities and relationships to database tables and columns. The expected behavior of each entity is typically represented as a sequence of statements to execute against the database. How would you represent the behavior of an entity? Data access design patterns provide some guidance on this; rest assured that whatever patterns suggested are to be implemented with concrete code. The CommandCollection property of a table adapter is the internal data structure that lists the statements used to hard-code an expected behavior on a table.

 
Figure 4: Tasks to extend and customize a table adapter.
So if you want to add a behavior to the table that represents the Customers table, you need to add a new T-SQL statement to the CommandCollection property and then make it public via a new method on the CustomersTableAdapter class. Visual Studio 2005 provides a simple wizard to do all of this. You trigger the wizard by choosing one of the tasks defined on the table adapter designer, as in Figure 4.

When you select a table adapter component in Visual Studio 2005, it displays a smart tag. After clicking it, Visual Studio displays a menu as in the figure. If you select to edit queries, you get a graphical representation of all the tables in the current data set component (see Figure 5.)

 
Figure 5: Table relationships and table adapters in the Visual Studio 2005 Data Designer.
To edit an existing query, you select the task in the table adapter task list and edit the properties in the Properties window. To add a new task, you select Add Query and follow the instructions of the subsequent wizard. The wizard ultimately guides you through the steps required to define a new command, be it a query or an update statement. When you're done, you see an additional entry in the task list and some changes to the underlying code.

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