Thursday, 3 April 2014

KeyedCollection

During a talk at DevWeek 2014 I discovered the KeyedCollection<TKey,TItem> class. The class is in the System.Collections.ObjectModel namespace, and is an alternative to the Dictionary<TKey,TValue> for items where the key is embedded in the item itself. The following code sample shows where it pays dividends to use a KeyedCollection over a Dictionary.

The following program creates a dictionary of Person objects; it uses the Id of the Person as the key, and the person itself as the value:


Bearing in mind that this is a contrived example, there is a problem with this code; having to specify the key manually has lead to a typo (didn't you spot it?):


If the key to use for your collection item can be derived from the value, the KeyedCollection allows you to do that.
The KeyedCollection class is abstract, so you need to inherit from it. Having done so, you can override the GetKeyForItem method; this is the method that the framework calls to obtain the key of the item:


Items can then be added to the collection as you would any collection, either using the Add method, or an object initialiser as I've done here:


When retrieving an item, the KeyedCollection behaves like a Dictionary rather than a List, because you access items by their key rather that their position in the collection:


See more about the KeyedCollection class on MSDN: http://msdn.microsoft.com/en-us/library/ms132438(v=vs.110).aspx