RichTextBox save as picture

FileStream pngfs = new FileStream("C:\\1.png", FileMode.OpenOrCreate);
RenderTargetBitmap rtb = 
                new RenderTargetBitmap((int)richTextBox1.ActualWidth, (int)richTextBox1.ActualHeight, 
                    96.0, 96.0, PixelFormats.Default);
rtb.Render(richTextBox1);
BitmapEncoder be = new PngBitmapEncoder();
be.Frames.Add(BitmapFrame.Create(rtb));
be.Save(pngfs);
pngfs.Close();

This function can be extended to screenshots of other controls

See TIANFANG 's blog post