Saving a canvas to png C# wpf

Question

So I am trying to take a snapshot of my canvas in WPF C# so that I can save it out as a png. The image saves incorrectly at present as it is including the left and top margi

Answer 1

Replace the first four lines with these lines

Rect bounds = VisualTreeHelper.GetDescendantBounds(canvas);
double dpi = 96d;

RenderTargetBitmap rtb = new RenderTargetBitmap((int)bounds.Width, (int)bounds.Height, dpi, dpi, System.Windows.Media.PixelFormats.Default);

DrawingVisual dv = new DrawingVisual();
using (DrawingContext dc = dv.RenderOpen())
{
    VisualBrush vb = new VisualBrush(canvas);
    dc.DrawRectangle(vb, null, new Rect(new Point(), bounds.Size));
}

rtb.Render(dv);

I have followed this article http://mcleodsean.wordpress.com/2008/10/07/bitmap-snapshots-of-wpf-visuals/ (for more explanation) and able to save the canvas without margins.