Efficiently Generating SHA256 Checksum For Files Using C#

I’m building a file synchronization with the cloud application and I want to store a checksum with the file so that I can verify later that the file is what I think it is. I’m not a crypto guy so I searched around the internet for a solution.   I found lots of examples and settled on this one that uses SHA256 for the job.  I also found some comments saying that it would be more efficient to wrap