The problem was that xaml was wrong:
the correct syntax is:
<DataGrid x:Name="ProcessInputImages"
ScrollViewer.HorizontalScrollBarVisibility="Hidden" RowHeaderWidth="0" AutoGenerateColumns="False" IsReadOnly="True" SelectionMode="Single" SelectionUnit="Cell"
IsHitTestVisible="True" AllowDrop="True" cal:Message.Attach="[Event Drop] = [Action ObjectDropped($eventargs)]" >
....
note ] at the end of cal:Message.Attach="[Event Drop] = [Action ObjectDropped($eventargs)] which was missing.