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HelixViewport3D problems in COM Interop Control


I have been using the Helix Toolkit for about 6 months now and love it. However presently I am trying to generate a report from the HelixViewport3D.
   <helix:HelixViewport3D x:Name="helixViewer" IsHeadLightEnabled="True">         
      <ModelVisual3D x:Name="model">
                            <AmbientLight Color="#808080"/>
                <helix:SphereVisual3D Fill="Red"/>
        <Button Content="Button" HorizontalAlignment="Left" Height="52" Margin="10,10,0,0" VerticalAlignment="Top" Width="117" Click="Button_Click"/>
The button click event uses a printdialog to generate output.
  private void Button_Click(object sender, RoutedEventArgs e)
            PrintDialog dlg = new PrintDialog();

            dlg.PrintVisual(helixViewer.Viewport, "Test");
If I do this in WPF or in Windows forms all is well and the code generates a report output. However if I embed the control in a COM interop UserControl then reference this within a VB6 application the same call to PrintVisual crashes with a FileNotFoundException.

This appears to be specific to the HelixViewport3D control as a regular WPF Viewport3D prints correctly also even under interop.

Can someone please advise if there is a reason this might happen. and possibly what file I may be missing that is preventing it working only under interop conditions.