Credit Card Validation Check by Code

private bool CheckIsValidCreditCard(string CreditCard){   bool IsValidCreditCard = false;   int sum = 0;   int MultiplyDigit = 0;   for (int i = 0; i < CreditCard.Length; i++)   {      if (i % 2 == 0)      {         MultiplyDigit = Convert.ToInt32(CreditCard.Substring(i, 1)) * 2;         if (MultiplyDigit > 9)            sum += MultiplyDigit - 9;         else            sum += MultiplyDigit;