Render a "not visible" WPF controls to an bitmap image - wpf

Question

Render a WPF control to a bitmap image is not a trivial task as I found out today. As I know now dealing with the parent control margin is a problem, as Rick Strahl wrote in his blog
http://www.west-wind.com/weblog/posts/2007/Sep/10/Rendering-a-WPF-Container-to-Bitmap
So far I am able to create bitmaps of any control that is visible inside a window but what I really need to do is create bitmaps of invisible controls. I just create them in code - simples shapes like rectangle and ellipse - and want to save them as bitmaps to disk.
For me this turn out to be a personal nightmare. Since my ActualHeight and ActualWidth is always 0 I use Height and Width instead. But all I get is a empty image in the size of my control.
How can I create a bitmap of any control without adding it to a visible window?

Answer 1

New elements haven't had their layout performed. You need to call Measure and Arrange on the control before you render it.
Canvas c = new Canvas();
Rectangle r = new Rectangle
{
Fill = Brushes.Orange,
Width = 200,
Height = 100
};
Ellipse e = new Ellipse
{
Fill = Brushes.DodgerBlue,
Width = 100,
Height = 100
};
Canvas.SetLeft(e, 150);
c.Children.Add(r);
c.Children.Add(e);
var size = new Size(250,100);
c.Measure(size);
c.Arrange(new Rect(size));
RenderTargetBitmap bmp = new RenderTargetBitmap(250, 100, 96, 96, PixelFormats.Pbgra32);
bmp.Render(c);
PngBitmapEncoder enc = new PngBitmapEncoder();
enc.Frames.Add(BitmapFrame.Create(bmp));
using(var s = File.OpenWrite("image.png"))
enc.Save(s);