Credit Card Number validation C#

using System.Text;


public bool IsValidNumber(string cardNumber)

{

const string allowed = "0123456789";

int i;

StringBuilder cleanNumber = new StringBuilder();

for (i = 0; i < cardNumber.Length; i++)

{

if (allowed.IndexOf(cardNumber.Substring(i, 1)) >= 0)

cleanNumber.Append(cardNumber.Substring(i, 1));

}

if (cleanNumber.Length <> 16)

return false;

for (i = cleanNumber.Length + 1; i <= 16; i++)

cleanNumber.Insert(0, "0");

int multiplier, digit, sum, total = 0;

string number = cleanNumber.ToString();

for (i = 1; i <= 16; i++)

{

multiplier = 1 + (i % 2);

digit = int.Parse(number.Substring(i - 1, 1));

sum = digit * multiplier;

if (sum > 9)

sum -= 9;

total += sum;

}

return (total % 10 == 0);

}