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Sorting Custom Collections : Page 4

Did you know that the .NET Framework has no built-in functionality to sort custom type collections? This article provides you with an easy way to use T-SQL-like sort expressions to sort your custom type collections and explains how this great utility works under the hood.




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Using SortableCollectionBase
Now that you know how a multiple-property comparer is written and how to create them dynamically, it is time to discuss how to make practical use of them in an application. You could create a utility method that takes a collection and a sort expression that would in turn use the GetMultiComparer method with the ArrayList's Sort method to sort the given collection. Doing so would require that all of your collections either expose a public Sort method like the ArrayList's or a public accessor for the Collection's underlying ArrayList. I suggest adding the SortableCollectionBase type to your libraries and deriving collections that you want to sort from that type. This provides familiar usage, like the DataView, that allows you to specify the sort expression on the collection type itself; it is also similar to the ArrayList in that respect. This suggestion also promotes good object-oriented design, since the action is taking place on the collection object. Use it as your collection base instead of System.Collections.CollectionBase.

In fact, I would recommend that you create a CollectionBase type in your own class hierarchy, deriving all of your collection types from that type and in turn deriving your CollectionBase from SortableCollectionBase. The reason for this is that if you decide you don't like my SortableCollectionBase, something better comes out in .NET v2, or I or someone else comes out with a better base class, you can more easily change the base of all your collections. Of course, you will have to retain the methods that SortableCollectionBase provides (primarily GetNewView and Sort) in your new base, if you don't want to update all usage of your collection types that use these methods. You could, for instance, add the Sort method to your CollectionBase and have it call any new sort method you like (or even none at all). In any case, using your own CollectionBase is a good practice in general and will likely reduce maintenance in the long run.

Regardless of how you are deriving from SortableCollectionBase, doing so will provide you with the Sort method that simply takes a T-SQL-like sort expression (without the ORDER BY clause), and it also provides you with the GetNewView methods that will create a shallow copy of the current collection. The benefit of GetNewView is that you can have multiple views of the same collection of objects, with possibly different sorts, much like the DataView provides different views of the DataTable. It also gives you a thread-safe copy of the collection, which is useful if other threads might be manipulating the source collection at the same time. I referred to GetNewView methods because there are three overloads. The first, which takes no parameters, will return a shallow copy SortableCollectionBase. The other two return an IList and take a System.Type parameter that should be the type of a collection that derives from SortableCollectionBase. The point of this abstraction is to make casting easier. The third overload also takes a sort expression that it will use to sort the new shallow copy, so you can get a new view with a different sort in one statement, which is quite useful for, e.g., binding to a data control. You could further extend the GetNewView method to also allow for filtering, extending the DataView analogy.

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