The System.Web.Compilation.ResourceExpressionBuilder class creates code to retrieve resource values when the page is executed. A resource file typically contains information localized for a particular language or culture.

A resource expression takes the form <%$ Resources: ClassKey, ResourceKey %> within the page. The part of the expression before the colon (:) designates the type of expression builder to use, and the part after the colon signifies the class name and resource key. The preceding expression would retrieve the following value from a file named ClassKey.resx:

<data name="ResourceKey"><value xml:space="preserve">Hello!</value></data>

When the page parser encounters an expression with the Resources prefix, it creates an instance of the System.Web.Compilation.ResourceExpressionBuilder class. The System.Web.Compilation.ResourceExpressionBuilder class either evaluates the expression or generates code to return a value for the expression when the page is executed.

If the expression is encountered in a page that will be compiled, the System.Web.Compilation.ResourceExpressionBuilder object generates code that retrieves the specified value from the resource file. If the expression is encountered in a page that will not be compiled, the System.Web.Compilation.ResourceExpressionBuilder object returns the value from the resource file when the page is parsed.