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Remember, the EmployeeDB class uses error-handling blocks to make sure connections are properly closed, but it doesn t catch exceptions. (Best design practices are to let the exception notify the web page, which can then decide how best to inform the user.) You can handle errors with the ObjectDataSource in the same way you handle them with the SqlDataSource first, handle the Selected, Inserted, Updated, or Deleted event; second, check for an exception; and third, mark it as handled. For more information, see the Handling Errors section earlier in the chapter.

Figure 9-1. Single-value data binding in various controls You ll see data binding expressions again when you create templates for more advanced controls in 10.

The data source controls expose a rich event model. Events tend to fall into two categories. Events ending in ing such as ObjectCreating occur while a task is underway and give you the chance to cancel or customize what s happening. Events ending in ed such as ObjectCreated occur when the task is finished and are suitable for logging the action, synchronizing other controls, and handling errors.

A key part of extending the data source controls takes place through event handling. For example, by default the ObjectDataSource is able to create your custom data access class only if it provides a zero no-argument constructor. However, you can extend the ObjectDataSource to work with data access classes that don t meet this requirement by writing code that reacts to the ObjectDataSource.ObjectCreating event. The current EmployeesDB class retrieves the database connection string directly from the web.config file, as shown here: Private connectionString As String Public Sub New() connectionString = WebConfigurationManager.ConnectionStrings("Northwind").ConnectionString End Sub However, you might want to add another constructor that lets the web page supply a specific connection string of its choosing: Public Sub New(ByVal connectionString As String) Me.connectionString = connectionString End Sub To force the ObjectDataSource to use this constructor, you need to handle the ObjectCreating event, create the EmployeeDB instance yourself, and then assign it to the data source using the ObjectDataSourceEventArgs: Protected Sub sourceEmployees_ObjectCreating(ByVal sender As Object, ByVal e As ObjectDataSourceEventArgs) e.ObjectInstance = New DatabaseComponent.EmployeeDB("...") End Sub Clearly, you could perform more complex initialization in the ObjectCreating event. For example, you could call an initialization method, choose to instantiate one of several derived classes, and so on.

Data binding expressions are always wrapped in the <%# and %> characters. ASP.NET 2.0 also adds support for different types of expressions, commonly known as $ expressions because they incorporate the $ character. Technically, a $ expression is a code sequence that you can add to an .aspx page and that will be evaluated by an expression builder when the page is rendered. The expression builder processes the expression and replaces it with a string value in the final HTML. Currently, ASP.NET includes a built-in expression builder that allows you to extract custom application settings and connection string information from the web.config file. For example, if you want to retrieve an application setting named appName from the <appSettings> portion of the web.config file, you can use the following expression: <asp:Literal Runat="server" Text="<%$ AppSettings:appName %>" /> Several differences exist between $ expressions and data binding expressions: Data binding expressions start with the <%# character sequence, and $ expressions use <%$. Unlike data binding expressions, you don t need to call the DataBind() method to evaluate $ expressions. Instead, they re always evaluated when the page is rendered.

The data source controls expose a rich event model. Events tend to fall into two categories. Events ending in ing such as ObjectCreating occur while a task is underway and give you the chance to cancel or customize what s happening. Events ending in ed such as ObjectCreated occur when the task is finished and are suitable for logging the action, synchronizing other controls, and handling errors.

You can also react to the ObjectDisposing event to perform cleanup. The ObjectDisposing event is fired just before the data access object is released (before the page is served). Usually, you won t need to use the ObjectDisposing event because a better alternative exists place your cleanup code in a dedicated Dispose() method inside your data access class. As long as you implement IDisposable, the ObjectDataSource will automatically call your Dispose() method. (To get a painless implementation of IDisposable for free, just derive your data access class from the System.ComponentModel.Component class and override the Dispose() method.)