How to Implement a Generic OrderedDictionary in .NET 3.5?

Implementing a Generic OrderedDictionary

While a generic implementation of OrderedDictionary may not be available in .NET 3.5, creating one isn't overly complex. We can leverage a KeyedCollection for storage and implement various methods to sort items like List does, effectively creating a hybrid of IList and IDictionary.

Interface

public interface IOrderedDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IOrderedDictionary
{
    // ...
}

Implementation

public class OrderedDictionary<TKey, TValue> : IOrderedDictionary<TKey, TValue>
{
    private KeyedCollection2<TKey, KeyValuePair<TKey, TValue>> _keyedCollection;
    
    // ...
}

Helper Classes

public class KeyedCollection2<TKey, TItem> : KeyedCollection<TKey, TItem>
{
    private Func<TItem, TKey> _getKeyForItemDelegate;

    // ...
}

public class Comparer2<T> : Comparer<T>
{
    private readonly Comparison<T> _compareFunction;

    // ...
}

public class DictionaryEnumerator<TKey, TValue> : IDictionaryEnumerator, IDisposable
{
    // ...
}

Tests

[TestClass]
public class OrderedDictionaryTests
{
    // ...
}

These tests demonstrate the various capabilities of the implemented OrderedDictionary.

Conclusion

While .NET may not provide a generic OrderedDictionary implementation, creating your own with these resources is a viable solution for maintaining both key-based lookup speed and insertion order.

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