I was not able to get the attributes filled in a datagrid
How is set the ItemsSource property of your datagrid?
Hw are set the columns?
Did you look at this sample : http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#SpatialQuery ?
The result of the query task is displayed in a datagrid, and when a row is selected, the feature is selected in the map.
Is it what you want to do?