Silverlight and ADO.NET Data Service Operations

In building my Silverlight RC example using ADO.NET Data Services for Entity Framework and NHibernate I ran into what I think is a common pattern.  I am writing an editor for XBox game data. The model for this data uses decorator tables in the database which are modeled as a common “Product” class and derived “Game”, “Console” and “Accessory” classes.  In the application I am using paging to only look at fifty results at once. This works fine on both sides.

But one of the pieces of information I wanted was a list of all the Game Genre.  This became problematic as ADO.NET Data Services wanted me to retrieve all 880 games in order to get a list of these Genres (of which there are only 20 some odd). The whole idea of using paging is go avoid the huge overhead of bringing down the whole entity. Interestingly when I executed a LINQ query that used projection into non-entities, the query wasn’t supported as projection isn’t allowed in the ADO.NET Data Services URI model (which the client uses).

What ADO.NET Data Services does allow is to create Service Operations (e.g. WebGet or WebInvoke) on the Data Service to extend the model for specific cases. There are some limitations (must return IEnumerable<T>, IQueryable<T> or void) but this works pretty well. The difference between returning IEnumerable and IQueryable is whether system queries can be applied to them.  Returning a fixed list (my need) meant to return a IEnumerable<T> list. Intersting that ADO.NET Data Services support returning an IEnumerable<T> of primitive types.  For example my operation was spec’d as:

[WebGet]
public IEnumerable<string> Genres()
{
}

This works and returns a simple XML file with the primitive values.  But alas the Silverlight Data Service Client doesn’t support non-entities. I tried using the DataServiceContext.BeginExecute() to call this service and it threw an exception that it couldn’t materialize non-entity classes. Hrmph!

This was a case where adding a quick web service call to get this data on the server and return it would have been the easy road, but that’s not how I roll, is it?

After confirming this behavior with the Data team, I decided to write an extension to the DataServiceContext class to support this.  In this little piece of code, the same pattern of calling DataServiceContext.BeginXXX is used. To make this work I simply use the HttpWebRequest class to do a simple GET to the server’s URI syntax and use LINQ to XML to convert the data into the simple types (String in my case).

I’ve started to help out with some new ideas on the CodePlex SilverlightContrib project so I thought this was the perfect place for this code.  Its not packaged up yet in a build (and probably won’t be until sometime after RTW ships) but if you want to grab it you can grab the latest checkin I made:

SilverlightContrib ChangeSet 41005

I’ll be shipping this new demo as soon as RTW ships (its not done yet). Look for the announcement here.