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Unlike data binding expressions, $ expressions can t be inserted anywhere in a page. Instead, you need to wrap them in a control tag and use the expression result to set a control property. That means if you just want to show the result of an expression as ordinary text, you need to wrap it in a Literal tag (as shown in the previous example). (The Literal control outputs its text to plain, unformatted HTML.) The first part of a $ expression indicates the name of the expression builder. For example, the AppSettings:appName expression works because a dedicated AppSettingsExpression builder is registered to handle all expressions that begin with AppSettings. Similarly, ASP .NET includes a ResourceExpressionBuilder for inserting resources (see 17) and a ConnectionStringExpressionBuilder that retrieves connection information from the <connectionStrings> section of the web.config file. Here s an example that uses the ConnectionStringExpressionBuilder: <asp:Literal Runat="server" Text="<%$ ConnectionStrings:Northwind %>" /> Displaying a connection string isn t that useful. But this technique becomes much more useful when you combine it with the SqlDataSource control you ll examine later in this chapter, in which case you can use it to quickly supply a connection string from the web.config file: <asp:SqlDataSource ConnectionString="<%$ ConnectionStrings:Northwind %>" ... /> Technically, $ expressions don t involve data binding. But they work in a similar way and have a similar syntax.

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

Earlier, you saw how you could use the SqlDataSource to execute parameterized commands. The same feat is possible with the ObjectDataSource, if you provide a suitable select method that accepts one or more parameters. You can then map each parameter to a control value, query string argument, and so on. To try this, you can use the EmployeeDB.GetEmployee() method, which retrieves a single employee by ID number. Here s the method declaration:

One of the most innovative features of $ expressions is that you can create your own expression builders that plug into this framework This is a specialized technique that, while impressive, isn t always practical As you ll see, custom $ expressions make the most sense if you re developing a feature that you want to use to extend more than one web application For example, imagine you want a way to create a custom expression builder that allows you to insert random numbers You want to be able to write a tag such as this to show a random number between 1 and 6: <asp:Literal Runat="server" Text="<%$ RandomNumber:1,6 %>" /> Unfortunately, creating a custom expression builder isn t quite as easy as you probably expect The problem is how the code is compiled.

a painless implementation of IDisposable for free, just derive your data access class from the System.ComponentModel.Component class and override the Dispose() method.)

When you compile a page that contains an expression, the expression evaluating the code also needs to be compiled with it However, you don t want the expression to be evaluated at that point instead, you want the expression to be reevaluated each time the page is requested To make this possible, your expression builder needs to generate a segment of code that performs the appropriate task The technology that enables this is CodeDOM (Code Document Object Model) a model for dynamically generating code constructs Every expression builder includes a method named GetCodeExpression() that uses CodeDOM to generate the code needed for the expression In other words, if you want to create a RandomNumberExpressionBuilder, you need to create a GetCodeExpression() method that uses CodeDOM to generate a segment of code for calculating random numbers Clearly, it s not that straightforward and for anything but trivial code, it s quite lengthy.