Checking to see if an image is Blank in C#

If you can tolerate the chance of getting it wrong, the approach seems fine; I have done something very similar in my case, although I always had a visual confirmation to deal with errors.

For the performance, the key open question is how you are getting the pixels to test. If you are using Bitmap.GetPixel, you are bound to have performance problems. (Search for "Bitmap.GetPixel slow" in Google to see lots of discussion.)

Far better performance will come from getting all the pixels at once and then looping over them. I personally like Bob Powell's LockBits discussion for clarity and completeness. With that approach, checking all of the pixels may well be reasonable depending on your performance needs.

If you're using System.Drawing.Bitmap you can speed up things up (substantially), by:

1. Not using GetPixel to access the pixels, use LockBits and UnlockBits to copy the image bitmap to regular memory. See the examples on the MSDN documentation for usage.
2. Not calling the Width, Height or Size properties in for loop. Call Size once, store the values in a local variable and use those in the loop.

Notes:

1. When using System.Drawing.Bitmap your image may be in device memory and accessing it may be time consuming.
2. I don't remember whether loading an image into a Bitmap already converts it to RGB format as other formats are more difficult to work with, but if that is not the case you can create an RGB Bitmap of the same size as your original image, get it's Graphic object (Graphics.FromImage) and use DrawImage to draw the original image in the RGB bitmap.

Edit: Beat to the punch by DocMax.

In any case for speed you can also try using alternative libraries such as the excellent FreeImage which includes C# wrappers.