Multiple keys with KeyedCollection

DoubleKeyedCollection.cs

namespace Diuturnal.Utility {

  [Serializable]
  public abstract class DoubleKeyedCollection<TKey, TSecondKey, TItem>
    : KeyedCollection<TKey, TItem> {

    private Dictionary<TSecondKey, TKey> _secondKeyIndex =
      new Dictionary<TSecondKey, TKey>();

    public TItem GetItem(TSecondKey secondKey) {
      return this[_secondKeyIndex[secondKey]];
    }

    protected abstract TSecondKey GetSecondKeyForItem(TItem item);

    protected override void InsertItem(int index, TItem item) {
      base.InsertItem(index, item);
      AddSecondKeyIndex(this.GetSecondKeyForItem(item),
        this.GetKeyForItem(item));
    }

    protected override void RemoveItem(int index) {
      RemoveSecondKeyIndex(this.GetSecondKeyForItem(this[index]));
      base.RemoveItem(index);
    }

    protected override void SetItem(int index, TItem item) {
      RemoveSecondKeyIndex(this.GetSecondKeyForItem(this[index]));
      base.SetItem(index, item);
      AddSecondKeyIndex(this.GetSecondKeyForItem(item),
        this.GetKeyForItem(item));
    }

    protected override void ClearItems() {
      base.ClearItems();
      _secondKeyIndex.Clear();
    }

    private void AddSecondKeyIndex(TSecondKey secondKey, TKey key) {
      _secondKeyIndex.Add(secondKey, key);
    }

    private void RemoveSecondKeyIndex(TSecondKey secondKey) {
      _secondKeyIndex.Remove(secondKey);
    }
  }
}