private string ComputeMD5Checksum(string path)
             {
                 using (FileStream fs = System.IO.File.OpenRead(path))
                 {
                     MD5 md5 = new MD5CryptoServiceProvider();
                     byte[] fileData = new byte[fs.Length];
                     fs.Read(fileData, 0, (int)fs.Length);
                     byte[] checkSum = md5.ComputeHash(fileData);
                     string result = BitConverter.ToString(checkSum).Replace("-", String.Empty);
                     return result.ToLower();
                 }
             }

Возникнут проблемы при работе с файлами большого объема.