title: How to work out if a file has been modified?
body: <p>I'm writing a back up solution (of sorts). Simply it copies a file from location C:\ and pastes it to location Z:\</p>
  
  <p>To ensure the speed is fast, before copying and pasting it checks to see if the original file exists. If it does, it performs a few 'calculations' to work out if the copy should continue or if the backup file is up to date. It is these calculations I'm finding difficult.</p>
  
  <p>Originally, I compared the file size but this is not good enough because it would be very possible to change a file and it to be the same size (for example saving the character C in notepad is the same size as if I saved the Character T). </p>
  
  <p>So, I need to find out if the modified date differs. At the moment, I get the file info using the <code>FileInfo</code> class but after reviewing all the fields there is nothing which appears to be suitable. </p>
  
  <p>How can I check to ensure that I'm copying files which have been modified?</p>
  
  <p><strong>EDIT</strong>
   I have seen suggestions on SO to use MD5 checksums, but I'm concerned this may be a problem as some of the files I'm comparing will be up to 10GB</p>