question

You can't just cast an object (assuming "value" is the item in question) as a Storage.Slot.Item.

You first construct the Storage.Item class representation of the item. From there you can access the current Slot.Item or the assigned Slot.Item. The current value being the slot the item is in at the moment, while the assigned slot could be a different slot the item is slated to be moved to.

Storage.Item storageItem = Storage.Item(value);
return storageItem.currentSlotItem.pickRank;