Rect rect = new Rect(canvas.RenderSize);
RenderTargetBitmap rtb = new RenderTargetBitmap((int)rect.Right,
  (int)rect.Bottom, 96d, 96d, System.Windows.Media.PixelFormats.Default);
rtb.Render(canvas);
//endcode as PNG
BitmapEncoder pngEncoder = new PngBitmapEncoder();
pngEncoder.Frames.Add(BitmapFrame.Create(rtb));
 
//save to memory stream
System.IO.MemoryStream ms = new System.IO.MemoryStream();