Why Is There No Generic OrderedDictionary in .NET 3.5, and How Can I Implement One?

Can't Find Generic Implementation of OrderedDictionary?

.NET 3.5 doesn't provide a generic implementation of OrderedDictionary (from the System.Collections.Specialized namespace).

There are known implementations that can provide this functionality, but it's unclear why this implementation isn't included out-of-the-box and whether it will be included in .NET 4.0.

Implement Your Own

Implementing a generic OrderedDictionary isn't complex, but can be time-consuming. Here's a solution using a KeyedCollection for internal storage:

public interface IOrderedDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IOrderedDictionary {
    new TValue this[int index] { get; set; }
    new TValue this[TKey key] { get; set; }
    new int Count { get; }
    new ICollection<TKey> Keys { get; }
    new ICollection<TValue> Values { get; }
    new void Add(TKey key, TValue value);
    new void Clear();
    void Insert(int index, TKey key, TValue value);
    int IndexOf(TKey key);
    bool ContainsValue(TValue value);
    bool ContainsValue(TValue value, IEqualityComparer<TValue> comparer);
    new bool ContainsKey(TKey key);
    new IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator();
    new bool Remove(TKey key);
    new void RemoveAt(int index);
    new bool TryGetValue(TKey key, out TValue value);
    TValue GetValue(TKey key);
    void SetValue(TKey key, TValue value);
    KeyValuePair<TKey, TValue> GetItem(int index);
    void SetItem(int index, TValue value);
}

public class OrderedDictionary<TKey, TValue> : IOrderedDictionary<TKey, TValue> {

    private KeyedCollection2<TKey, KeyValuePair<TKey, TValue>> _keyedCollection;

    public TValue this[TKey key] {
        get {
            return GetValue(key);
        }
        set {
            SetValue(key, value);
        }
    }

    public TValue this[int index] {
        get {
            return GetItem(index).Value;
        }
        set {
            SetItem(index, value);
        }
    }

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