Try this answer:
public void ExportToPng(Uri path, Canvas surface)
{
if (path == null) return;
// Save current canvas transform
Transform transform = surface.LayoutTransform;
// reset current transform (in case it is scaled or rotated)
surface.LayoutTransform = null;
// Get the size of canvas
Size size = new Size(surface.Width, surface.Height);
// Measure and arrange the surface
// VERY IMPORTANT
surface.Measure(size);
surface.Arrange(new Rect(size));
// Create a render bitmap and push the surface to it
RenderTargetBitmap renderBitmap =
new RenderTargetBitmap(
(int)size.Width,
(int)size.Height,
96d,
96d,
PixelFormats.Pbgra32);
renderBitmap.Render(surface);
// Create a file stream for saving image
using (FileStream outStream = new FileStream(path.LocalPath, FileMode.Create))
{
// Use png encoder for our data
PngBitmapEncoder encoder = new PngBitmapEncoder();
// push the rendered bitmap to it
encoder.Frames.Add(BitmapFrame.Create(renderBitmap));
// save the data to the stream
encoder.Save(outStream);
}
// Restore previously saved layout
surface.LayoutTransform = transform;
}
This answer was copied here for convenience from [this page.]<====LINK DEAD(http://denisvuyka.wordpress.com/2007/12/03/wpf-diagramming-saving-you-canvas-to-image-xps-document-or-raw-xaml/)