How do I validate a UPC or EAN code?

Question

I need a C# .NET function to evaluate whether a typed or scanned barcode is a valid Global Trade Item Number (UPC or EAN).

The last digit of a bar code number is a computer Check Digit which makes sure the bar code is correctly composed. GTIN Check Digit Calculator

Answer 1

public static bool IsValidGtin(string code)
{
    if (code != (new Regex("[^0-9]")).Replace(code, ""))
    {
        // is not numeric
        return false;
    }
    // pad with zeros to lengthen to 14 digits
    switch (code.Length)
    {
        case 8:
            code = "000000" + code;
            break;
        case 12:
            code = "00" + code;
            break;
        case 13:
            code = "0" + code;
            break;
        case 14:
            break;
        default:
            // wrong number of digits
            return false;
    }
    // calculate check digit
    int[] a = new int[13];
    a[0] = int.Parse(code[0].ToString()) * 3;
    a[1] = int.Parse(code[1].ToString());
    a[2] = int.Parse(code[2].ToString()) * 3;
    a[3] = int.Parse(code[3].ToString());
    a[4] = int.Parse(code[4].ToString()) * 3;
    a[5] = int.Parse(code[5].ToString());
    a[6] = int.Parse(code[6].ToString()) * 3;
    a[7] = int.Parse(code[7].ToString());
    a[8] = int.Parse(code[8].ToString()) * 3;
    a[9] = int.Parse(code[9].ToString());
    a[10] = int.Parse(code[10].ToString()) * 3;
    a[11] = int.Parse(code[11].ToString());
    a[12] = int.Parse(code[12].ToString()) * 3;
    int sum = a[0] + a[1] + a[2] + a[3] + a[4] + a[5] + a[6] + a[7] + a[8] + a[9] + a[10] + a[11] + a[12];
    int check = (10 - (sum % 10)) % 10;
    // evaluate check digit
    int last = int.Parse(code[13].ToString());
    return check == last;
}