A new feature in ASP.NET 2.0 has been introduced, the ExpressionBuilder. The ExpressionBuilder allows declarative binding for control properties for data that is coming either from the ConnectionStrings and AppSettings section of the web configuration file or from the web application resource files.

The way you add a declarative expression to a control property is as follows:

Listing 1

<asp:Label ID="lblPath" runat="server" Text='<%$ AppSettings: WebPath %>' />

As you can see above, we have assigned the Text property of the Label control a declarative expression which shall retrieve the WebPath key from the AppSettings section of the web.config file of the ASP.NET web application.

When the page is being parsed, if the parser passes by such a declarative expression, it will create a new instance of the AppSettingsExpressionBiulder and then evaluate the expression. Evaluating the expression in this case is by taking the right hand side of the declarative expression which is WebPath and then retrieving the value of that key from the web configuration file. Finally, it binds the value of that key to the Text property of the Label control.

In this article we will cover the different built-in ExpressionBuilders that shipp with the ASP.NET 2.0 and build a custom ExpressionBuilder to bind the ASP.NET controls to a separate XML settings files.