A function with this signature can be used for the AddValidationCallback( ) method of the HttpCachePolicy class. Using this method gives you the opportunity to check other dependencies that a cached page may have and either allow the page to be served or invalidate it and require the page to be recompiled. To invalidate a page, set the validationStatus parameter to HttpValidationStatus.Invalid.
public delegate void HttpCacheValidateHandler(HttpContext context, object data, ref HttpValidationStatus validationStatus);