You can include a LegendExtension from this thread: http://forums.arcgis.com/threads/18999-How-do-I-get-the-selected-LegendItem and access the selected LegendItemViewModel.
private void Button_Click(object sender, System.Windows.RoutedEventArgs e)
{
var result = this.MyLegend.SelectedItems();
foreach (var item in result)
MessageBox.Show(item.Description);
}
Another way is to parse the json.
private void ArcGISDynamicMapServiceLayer_Initialized(object sender, System.EventArgs e)
{
ArcGISDynamicMapServiceLayer layer = sender as ArcGISDynamicMapServiceLayer;
foreach (var l in layer.Layers)
{
//TODO use this sub layer url
//string.Format("{0}/{1}/f=json", layer.Url, l.ID));
}
}
http://forums.silverlight.net/forums/p/169016/385411.aspx