Write out a digest file.
/*
* System.IO
* System.Security.Cryptography
*/
string path = GetPathToFileYouWantToHash();
byte[] data = File.ReadAllBytes(path);
SHA512Managed sha = new SHA512Managed();
byte[] digest = sha.ComputeHash(data);
// http://stackoverflow.com/a/623134/155167
string hex = String.Concat(digest.Select(b => b.ToString("x2")));
string digestFilePath = GetPathForDigestFile();
File.WriteAllText(digestFilePath, hex, Encoding.ASCII);