.net collection for fast insert/delete

Question

I need to maintain a roster of connected clients that are very shortlived and frequently go up and down. Due to the potential number of clients I need a collection that supports

Answer 1

Well, how much do you need to query it? A linked-list has fast insert/delete (at any position), but isn't as quick to search as (for example) a dictionary / sorted-list. Alternatively, a straight list with a bit/value pair in each - i.e. "still has value". Just re-use logically empty cells before appending. Delete just clears the cell.

For reference types, "null" would do here. For value-types, Nullable.