Is there a general concrete implementation of a KeyedCollection?

Question

The System.Collections.ObjectModel.KeyedCollection class is a very useful alternative to System.Collections.Generic.Dictionary, especially when the key data is part of the object being stored or you want to be able to enumerate the items in order. Unfortunately, the class is abstract, and I am unable to find a general concrete implementation in the core .NET framework.

The Framework Design Guidlines book indicates that a concrete implementation should be provided for abstract types (section 4.4 Abstract Class Design). Why would the framework designers leave out a general concrete implementation of such a useful class, especially when it could be provided by simply exposing a constructor that accepts and stores a Converter from the item to its key:

public class ConcreteKeyedCollection<TKey, TItem> : KeyedCollection<TKey, TItem>
{
    private Converter<TItem, TKey> getKeyForItem = null;
    public ConcreteKeyedCollection(Converter<TItem, TKey> getKeyForItem)
    {
        if (getKeyForItem == null) { throw new ArgumentNullException("getKeyForItem"); }
        this.getKeyForItem = getKeyForItem;
    }
    protected override TKey GetKeyForItem(TItem item)
    {
        return this.getKeyForItem(item);
    }
}

Answer 1

There are concrete implementations, including (but not limited to):

• KeyedByTypeCollection<TItem> (I use this class frequently)
• MessageHeaderDescriptionCollection

To the spirit of your question, no there is no generic implementation and since I do not work for Microsoft I could only speculate. Since the concrete implementation is as easy as you've shown, I won't offer any speculation (as it would probably be wrong).

Answer 2

Here is the implementation I came up with

public class LookupKeyedCollection<TKey, TItem> : KeyedCollection<TKey, TItem>
{
    private Func<TItem, TKey> _getKeyFunc;

    public LookupKeyedCollection(Func<TItem, TKey> getKeyFunc)
    {
        _getKeyFunc = getKeyFunc;
    }

    //Required KeyedCollection implementation
    protected override TKey GetKeyForItem(TItem item)
    {
        return _getKeyFunc(item);
    }

    public bool TryGetValue(TKey key, out TItem item)
    {
        if (Dictionary == null)
        {
            item = default(TItem);
            return false;
        }

        return Dictionary.TryGetValue(key, out item);
    }

    public void AddOrUpdate(TItem item)
    {
        if (!Contains(item))
            Add(item);
    }

    public new bool Contains(TItem item)
    {
        return base.Contains(_getKeyFunc(item));
    }
}

The reasoning behind the methods can be mostly found in the following:

• http://www.devnewsgroups.net/group/microsoft.public.dotnet.framework/topic62318.aspx
• http://www.hardcodet.net/2008/03/numericallykeyedcollection