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SHA512 sha512 = SHA512.Create();                    

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SHA512Managed sha512 = new SHA512Managed();                    

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using (FileStream stream = File.OpenRead(file))
    {
        SHA512Managed sha = new SHA512Managed();
        byte[] hash = sha.ComputeHash(stream);
    }                    

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foreach(byte hashSubstring in sha512Hash) File.AppendAllText(pathFileList, Convert.ToString(hashSubstring, 16));                    

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static void Main(string[] args)
        {
            DateTime Time1 = DateTime.Now;
            string sFile = @"C:\test.txt";

            // Use a file as an input stream, but we can pretend that it is something like a response stream from 
            // a web request.
            using (SHA512 md5 = SHA512.Create())
            using (Stream input = File.OpenRead(sFile))
            {
                // small enough buffer to make us read the file in chunks
                byte[] buffer = new byte[BUFFER_SIZE];
                int bytesRead;

                // open the file, 
                while ((bytesRead = input.Read(buffer, 0, buffer.Length)) > 0)
                {
                    // hash each chunk at a time.
                    md5.TransformBlock(buffer, 0, bytesRead, null, 0);
                }

                // We need this, even though no data is hashed.  Do not
                // pass null for the buffer
                md5.TransformFinalBlock(buffer, 0, 0);

                byte[] hashVal = md5.Hash;

                // convert the hash value to a nice string, something like would see on a download site or 
                // get from the Microsoft hasing utility, fciv.exe.
                StringBuilder sbHash = new StringBuilder(hashVal.Length * 2);
                int idxMax = hashVal.Length;
                for (int idx = 0; idx < idxMax; ++idx)
                {
                    sbHash.Append(hashVal[idx].ToString("x2"));
                }

                Console.WriteLine(sbHash.ToString());
                DateTime Time2 = DateTime.Now;
                TimeSpan timeSpan = Time2 - Time1;
                Console.WriteLine(timeSpan.Milliseconds);
            }