在asp.net中管理缓存对象

I have a list of product that I have stored in asp.net cache but I have a problem in refreshing the cache. As per our requirement I want to refresh cache every 15 minutes but I want to know that if in the mean time when the cache is being refreshed if some user ask for the list of product then should he get error or the old list or he have to wait until the cache is refreshed.I have a list of product that I have stored in