How do I crop an image using C#?

Simpler than the accepted answer is this:

public static Bitmap cropAtRect(this Bitmap b, Rectangle r)
{
    using (var nb = new Bitmap(r.Width, r.Height))
    {
        using (Graphics g = Graphics.FromImage(nb))
        {
            g.DrawImage(b, -r.X, -r.Y);
            return nb;
        }
    }
}

and it avoids the "Out of memory" exception risk of the simplest answer.

Note that Bitmap and Graphics are IDisposable hence the using clauses.

EDIT: I find this is fine with PNGs saved by Bitmap.Save or Paint.exe, but fails with PNGs saved by e.g. Paint Shop Pro 6 - the content is displaced. Addition of GraphicsUnit.Pixel gives a different wrong result. Perhaps just these failing PNGs are faulty.