Verify National Provider Identifier

VerifyNPI.cs

/*
usage:
  bool? result = verifyNPI("1234567893");
  Console.WriteLine(result ?? false);
*/

public static bool? verifyNPI(string npi)
{
  if (!Regex.IsMatch(npi, @"^[0-9]{10}$"))
    return null;

  var count = 0;

  for (int i = 8; i >= 0; i--)
  {
    if (i % 2 == 0)
    {
      var tempNum = int.Parse(npi[i].ToString()) * 2;

      if (tempNum >= 10)
        count += (tempNum % 10) + 1;
      else
        count += tempNum;
    }
    else
    {
      count += int.Parse(npi[i].ToString());
    }
  }

  var checkDigit = (1006 - count) % 10;

  return npi[9].ToString() == checkDigit.ToString();
}