There are 3 steps in displaying the database data in a Silverlight application (and a ComboBox)
1. Access and query the database - in your case that is the Linq to SQL
2. Expose the data to the silverlight application - Web Service: Duplex, Restful, Sockets, etc
3. Consume the data on the Silvelight side.
The three steps are not directly related and depend on the specific scenario. In any case, when you have your data on the client side (in the silverlight app), it will be in the form of data object.
Here is help article on binding:
Our northwind example shows how you can access a database and populate it:
You can download the source for the demo from your client.net account.
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