Thread Safety

static class Extensions
{
    public static GraphicsContainer GetCopy(this GraphicsContainer gc)
    {
        var copy = new GraphicsContainer(gc.Width, gc.Height, gc.DpiX, gc.DpiY);

        using (var gr = copy.GetGraphics())
        {
            gr.DrawContainer(gc, 0, 0);
        }

        return copy;
    }

    public static void Save(this GraphicsContainer gc, string path)
    {
        using (var ig = new ImageGenerator(gc, PixelFormat.Format32bppArgb, RgbColor.Transparent))
        using (var writer = ImageWriter.Create(path))
        {
            Pipeline.Run(ig + writer);
        }
    }
}

var gc = new GraphicsContainer(width, height, dpiX, dpiY);

// GUI thread
lock(gc)
{
    using (var gr = gc.GetGraphics())
    {
        gr.FillRectangle(...);
        // other drawing
    }
}

// Background thread
GraphicsContainer copy = null;

lock(gc)
{
    copy = gc.GetCopy();
}

using (copy)
{
    copy.Save("out.png");
}