delete more than one row in datagrid in silverlight 3

I tried to delete more than one row in datagird in silverlight, using chechbox checked, and binding to itemsource. I have following code:
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Orderlist = (System.Collections.ObjectModel.ObservableCollection<SilverlightWebService.Order>)dgOrders.ItemsSource;
 System.Collections.ObjectModel.ObservableCollection<SilverlightWebService.Order> orderlist1 = new System.Collections.ObjectModel. ...