Is web.config or app.config cached in memory

Web.config (excluding external config files) is read when the application loads. Some config settings have a cascading behavior. For example, the system.web/authorization section can be overridden by configs at deeper levels.

ASP.NET monitors the web.config for changes. When it changes, the web application is forced to restart. Moral is that web.config settings are cached for the life of the application.