I'm looking for a data structure for working with a set of data which is most efficient to do the following : Check whether an item has been categorized or not. (The categorized and uncategorized set are disjoint sets). Get the category of an item. Get all the items in a particular category. Get all the uncategorized items. Remove a particular item from the data set. I was thinking of having a Dictionary<String, Set<String>> to hold all the items in a given category, but that doesn't solve 2.