public string GetMD5HashFromFile(string fileName)
{
  FileStream file = new FileStream(fileName, FileMode.Open);
  MD5 md5 = new MD5CryptoServiceProvider();
  byte[] retVal = md5.ComputeHash(file);
  file.Close();
 
  StringBuilder sb = new StringBuilder();
  for (int i = 0; i < retVal.Length; i++)
  {
    sb.Append(retVal[i].ToString("x2"));
  }
  return sb.ToString();
}