Pngbitmapencoder render empty image when use windowsservice project

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C#

WPF

controls

I Have a WindowsService project.In this service I wanna render an UserControl but png file is empty.

for test i write the following code to export a TextBlock but result is an empty png file.

protected override void OnCustomCommand(int command)
{
    Export();

    base.OnCustomCommand(command);
}

private void Export()
{
    var thread = new Thread(() =>
    {

        var grid = new Grid();
        grid.Children.Add(new TextBlock() { Text = "ExportTest", FontSize = 60, 
                          Background = Brushes.Red, Foreground = Brushes.Blue });
        double widthg = grid.Width > 0 ? grid.Width : 1024;
        double heightg = grid.Height > 0 ? grid.Height : 768;
        grid.Measure(Size.Empty);
        grid.Measure(new Size(widthg, heightg));
        grid.Arrange(new Rect(0, 0, widthg, heightg));
        grid.UpdateLayout();
	

        Size size = new Size(widthg, heightg);
        RenderTargetBitmap result = new RenderTargetBitmap((int)size.Width, 
                                   (int)size.Height, 96, 96, PixelFormats.Pbgra32);
        DrawingVisual drawingvisual = new DrawingVisual();
        using (DrawingContext context = drawingvisual.RenderOpen())
        {
             context.DrawRectangle(new VisualBrush(grid), null, new Rect(new Point(), 
                                  size));
             context.Close();
        }
        result.Render(drawingvisual);
        using (var fs = new FileStream(@"..\ExportTest.png", FileMode.Create))
        {
             PngBitmapEncoder encoder = new PngBitmapEncoder();
             encoder.Frames.Add(BitmapFrame.Create(result));
             encoder.Save(fs);
        }

    });

    thread.SetApartmentState(ApartmentState.STA);
    thread.Start();
    thread.Join();
}

Note: when i run the export function in wpf project, it works perfect.

there is my project:
http://www.mediafire.com/file/aij5sg4zx80yc73/ExportProject.rar
any body can help me?

What I have tried:

I Searched but found nothing. can any body help me?