Question

No one said that OrderedDictionary is having two copies of elements, one in a hashtable and other in a list, I can’t find complexity measurements at MSDN for OrderedList.

thanks

Answer 1

Have a look at

OrderedDictionary: A generic implementation of IOrderedDictionary

This implementation of an ordered
dictionary is very good at lookup
operations: the array allows O(1)
lookups by index, and the hashtable
allows O(1) lookups by key. However,
the necessity of keeping the array
synchronized with the hashtable means
that insert/delete operations have the
performance disadvantage of performing
those operations on an array (O(n) at
worst). There is also, of course, the
extra memory requirement of storing
both data structures. Because of these
disadvantages, OrderedDictionary
should only be used when insert/delete
operations will be minimal and there
is a need to efficiently access
elements by index and/or key.