Task: Create lower-level implementation of LinkedDictionary<TKey, TValue>

[NightOwl888]

The LinkedDictionary<TKey, TValue> is a composite type that internally is using a LinkedList<T> and Dictionary<TKey, TValue>. This is a stopgap to get the behavior we need, but we could achieve better performance by using a lower-level hashtable structure to manage the a single collection of items rather than having 2 collections.

The main differences between Dictionary<TKey, TValue> and LinkedDictionary<TKey, TValue> are:

• LinkedDictionary<TKey, TValue> maintains insertion order across edits
• LinkedDictionary<TKey, TValue> allows a null key

Update

.NET 9.0 now includes an OrderedDictionary<TKey, TValue> that should provide the correct functionality. It is MIT licensed, so there is no issue with copying it. We need to evaluate the collection to ensure it behaves correctly. It does not support null keys, which is why we will still need to bring it into our project and modify it. I believe we can simply return 0 for a null hash key and run the same lookup code, but if not check the source code of LinkedHashMap.java in Apache Harmony (which is Apache 2.0 licensed code).

We already have tests in J2N.Tests.xUnit to confirm it behaves as expected, however we should also review the tests Microsoft provides for OrderedDictionary<TKey, TValue> to determine whether there are any gaps that we missed.