How to get focused/targeted image?

After i added functionality to draw target/rectangle on preview, i received some emails with question how to grab part that exactly in rectagnle.
Ok you dont need anything special and any lib, for example you draw target with

Rect _rect

then to grab that part you need:
1. Grab full image

Bitmap bmp = new Bitmap(width, height);
System.Drawing.Imaging.BitmapData data = bmp.LockBits(new […]