Assembly: System.Web (in system.web.dll)
ASP.NET automatically evaluates expressions during page parsing using the ExpressionBuilder class. The ExpressionBuilderCollection collection, which is made up of the expressionBuilders elements contained in the compilation section of the configuration. The ExpressionBuilder contains specific values in key/value pairs.
A value is retrieved by including an expression of the form
<%$ ExpressionPrefix: ExpressionKey %>
within the page. The ExpressionPrefix maps the type of expression to be retrieved as either a common expression-builder type (that is, AppSettingsExpressionBuilder) or a custom expression builder type. The ExpressionKey specifies the key used to reference the returned value.
If you choose to implement a common expression-builder type, you must map the expression builder to the associated section name in the configuration. If you choose to implement a custom expression-builder type, you must map your expression builder to your custom expression-builder class, which must derive from the ExpressionBuilder class.