The AppSettingsExpressionBuilder.SupportsEvaluate property indicates whether the AppSettingsExpressionBuilder.EvaluateExpression(object, System.Web.UI.BoundPropertyEntry, object, ExpressionBuilderContext) method has been implemented in the class and can return a value for the expression. AppSettingsExpressionBuilder.EvaluateExpression(object, System.Web.UI.BoundPropertyEntry, object, ExpressionBuilderContext) must be implemented within an expression builder for an expression to be evaluated in a page that is not compiled. The System.Web.Compilation.AppSettingsExpressionBuilder class supports evaluation of an application setting in a page that is not compiled, so it always returns true.