What does LockBits/UnlockBits do in c#?

Question

I have a method in c# that the only thing it does its LockBits, and then UnlockBits, and the images(input/output, transformed to byte arrays) are different. The one from output has less 100 and something bytes than the one from the input. This happens only with .jpg files. And checking the files in HxD I came to the understanding that it´s removing a part of the header, the exif signature to be exact. But I don't know how and why.

Does someone know what this is doing?

Here's the code:

public Image Validate (image){
  BitmapData original = null;
  Bitmap originalBMP = null;
  try{
     originalBMP = image as Bitmap;
     original = originalBMP.LockBits(new Rectangle(0, 0, 
        originalBMP.Width, originalBMP.Height),
        ImageLockMode.ReadWrite,
        originalBMP.PixelFormat);
     originalBMP.UnlockBits(original);
  }catch{}

  return image;
}

Answer 1

Calling Bitmap.LockBits() followed by Bitmap.UnlockBits() does nothing.

The behavior you observe is because of loading a JPEG image, and then saving it again. JPEG uses a lossy algorithm. So what happens:

1. You load the JPEG from disk
2. The JPEG data gets decoded into individual pixels with color information, i.e. a bitmap
3. You save the bitmap again in the JPEG format, resulting in a different file than #1

You also potentially lose metadata that was present in the JPEG file in doing so. So yes, the file is different and probably smaller, because every time you do this, you lose some pixel data or metadata.

Lockbits/Unlockbits are used to allow the program to manipulate the image data in memory. Nothing more, nothing less. See also the documentation for those methods.