Find differences between images C#

Hi,

I am getting following error:

Bitmap region is already locked.

At the line:

bmData2 = bmp2.LockBits(new Rectangle(0, 0, bmp2.Width, bmp2.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

Can you tell me if I am doing anything wrong here?

Its VS2010, .Net4.0, c# windows application