This situation occurs when an assembly is used by both windows and XBAP application in a partial trust. The assembly might have permission when used through windows application and the same when used through XBAP application will throw a security exception. To prevent this situation, the permission for a file can be checked using the ‘Demand()’ method on the instance of the desired permission. If a file doesn’t have permission, the Demand() method throws a security exception, else it can be used by the partial trust application.