When Hoarding Dragon enters the battlefield, you may search your library for an artifact card, exile it, then shuffle your library.
When Hoarding Dragon dies, you may put the exiled card into its owner's hand.
11/17/2017 The artifact card you find is exiled face up. All players can see what it is.
11/17/2017 Hoarding Dragon’s two abilities are linked. The second ability refers only to the card exiled by that Hoarding Dragon’s first ability. In other words, each Dragon has its own hoard.
11/17/2017 If Hoarding Dragon dies before its first ability resolves, its second ability will trigger and do nothing. Then its first ability will resolve. If you choose to exile an artifact card from your library, it will be exiled indefinitely.