Ideas for categorized items with hierarchical relationships?

This is for a .NET control I'开发者_C百科m creating.

The control has an Items property that contains all items as a flat list. There is also a Categories property: each item belongs to one category. Each Category stores items under it both as a flat list and as a hierarchical tree. The flat lists should be sortable. Relationships must be made using a variety of constructors or adding/inserting to any of the collections above. So the problem is that adding a single thing will have to be added in several places automatically.

I want to provide for flexible ways to add items and categories in many ways, but I got totally stuck with the intricacies here. This is more complex than you would think at first (don't be quick to answer because I've spent hours trying all the 'obvious' solutions).

Where can I find similar implementations that can be inspirations?