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POIs there a general concrete implementation of a KeyedCollection?
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copied!<p>The <a href="http://msdn.microsoft.com/en-us/library/ms132439.aspx" rel="noreferrer">System.Collections.ObjectModel.KeyedCollection</a> class is a very useful alternative to <a href="http://msdn.microsoft.com/en-us/library/xfhwa508.aspx" rel="noreferrer">System.Collections.Generic.Dictionary</a>, 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.</p> <p>The <a href="http://rads.stackoverflow.com/amzn/click/0321545613" rel="noreferrer">Framework Design Guidlines</a> book indicates that a concrete implementation <em>should</em> 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 <a href="http://msdn.microsoft.com/en-us/library/kt456a2y.aspx" rel="noreferrer">Converter</a> from the item to its key:</p> <pre><code>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); } } </code></pre>

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