Adding a Data Layer Class

In most cases it's best to architect your applications to use a data access layer that encapsulates all the database interactions. This protects the database, simplifies calls from the presentation layer, and allows you to wrap the data access logic in business logic (or use a separate business tier altogether). VWD supports this commonly used architecture by allowing you to bind to an object as a data source.

In this sample you will build a simple data access layer and consume it via an ObjectDataSource control, binding it to a GridView control. This class will encapsulate methods exposed by a TableAdapter class that services a typed DataSet. This implementation will become much clearer as you proceed through the next few lessons.

You begin by adding a typed DataSet to your project. Typed DataSets are better to use than generic DataSets because they strongly type the objects unique to your data source based on the data source's schema (metadata). You thus have full IntelliSense support for your data objects such as tables, rows and columns, reducing guesswork and coding errors. You also get a performance boost because late binding is not used to access the data objects (e.g., field name look-up in a collection).

1.  In the Solution Explorer right-click the project and select Add New Item. In the Add New Item dialog select DataSet and call it "AuthorsDataSet.xsd". Click Add. When prompted to create an App_Code folder, click Yes.

ASP.NET 2.0 includes a suite of special folders. Among them is the App_Code folder. VWD monitors source code files in this folder, automatically compiling them and making the resulting binary accessible to code in the rest of your project. In this way it acts like the traditional bin folder, but you don't have to explicitly compile or add a reference to the resulting assembly.

When a typed DataSet is added to your project, VWD also adds a TableAdapter class to work with the DataSet. You will now configure the TableAdapter. This is similar to what you saw when configuring the SqlDataSource object.

2.  In the TableAdapter Configuration Wizard click Next twice. In the Enter a SQL Statement step, enter the SELECT statement you used in the previous lessons: "SELECT [au_id], [au_lname], [au_fname], [phone], [address], [city], [state], [zip], [contract] FROM [authors] WHERE ([state] = @state)". Click Next. You will notice that all options are selected. Click Next. Click Finish. Press CTRL+S to save the changes. (The AuthorsDataSetTableAdapters namespace is not available until you do so.) You should see the following in the Design window.

You have configured your typed DataSet and TableAdapter. Although you could work directly with these classes from the presentation layer, you will now add a custom data access class to simulate real-world n-tier architectural practice.

3. In the Solution Explorer right-click App_Code and select Add New Item. In the Add New Item dialog select Class and call it "DataAccess.vb" (or, if working in C#, "DataAccess.cs"). Click Add.

4. To the DataAccess class add the following method for filling the typed DataSet, removing the line breaks required for Web presentation.

Public Function GetAuthors(ByVal state As String) As AuthorsDataSet
    Dim authorsTableAdapter As New AuthorsDataSetTableAdapters.authorsTableAdapter
    Dim authorsData As New AuthorsDataSet()
    authorsTableAdapter.Fill(authorsData.authors, state)
    Return authorsData
End Function

Notice that this code provides no additional functionality over the TableAdapter class. In a real world scenario you would likely have additional custom logic wrapping this code, such as validation or business logic.