ADO.NET classes like DataTable, DataRow, etc are not available in Silverlight and for that reason a direct access to a database is impossible. One of the best ways to access a database in Silverlight is to use a web service as an intermediary that can query a database on behalf of your Silverlight appliation.
Since you already have a web service, you just need to modifity it in such a way that it returns data that can be consumed by Silverlight. Instead of returning a dataset your service might return objects that represents you database data. For example, if your data rows contain customer data your web service might return a list of customer objects.
There are quite a few ways to do that so I'm sending you several blog posts that describe in details how to consume data using WCF:
Creating Web Services for Silverlight
Silverlight + WCF + LINQ to SQL
Consiming WCF Services in Silverlight
LINQ + WCF + Silverlight
Service-Driven Apps With Silverlight 2 And WCF
Another resource that might be useful is our JobBoard integration example. It is a Silverlight sample application that showcases some of our Silverlight controls and also makes use of web services that expose data to Silverlight. The source code of JobBoard is available here
(you just need to be logged in at telerik.com).
Hope this helps.
the Telerik team
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