What is the need of OrderedDictionary, ListDictionary, and HybridDictionary?`

To complement Kyle's answer:

OrderedDictionary allows retrieval by key and index (it uses a hashtable and and array internally) but has a bigger overhead per item

ListDictionary has a linked list as its internal structure, it does not perform well for insertion and retrieval by key but preserves the original insert order

HybridDictionary is a ListDictionary if the dictionary does not contain many items and converts to a Hashtable if the number of items reaches a speficic limit (I personally think you should use Dictionary<,> instead of it since C#2)

In a nutshell:

• Dictionary - Well, a dictionary.

• ListDictionary - Used for small collections, typically less than 10 items

• HybridDictionary - Used when the collection size is unknown (switches implementations depending on the size of the collection)

• OrderedDictionary - The elements of an OrderedDictionary are not sorted by the key, unlike the elements of a SortedDictionary<TKey, TValue> class. You can access elements either by the key or by the index.