Why is only the visible part of the canvas captured when printing?

Question

WPF

When trying to print the contents of the canvas, only the visible part is captured. Everything below the vertical scroll is not printed. What can be done?

<ScrollViewer x:Name="preview_scroll" HorizontalAlignment="Left" Height="382" VerticalAlignment="Top" Width="592" Margin="20,20,0,0">
            <Canvas x:Name="preview_canvas" Height="379" Width="582" HorizontalAlignment="Left" VerticalAlignment="Top"/>
</ScrollViewer>

Seal

try
            {
                PrintDialog dialog = new PrintDialog();

                if (dialog.ShowDialog() != true)
                    return;
                dialog.PrintVisual(preview_canvas, "Print");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Print Screen", MessageBoxButton.OK, MessageBoxImage.Error);
            }