EAN / UPC validation

UpcCode.cs

public class UpcCode
{
    public static bool IsValid(string code)
    {
        if (string.IsNullOrEmpty(code)) return false;
        if (!code.All(char.IsDigit)) return false;
        if (code.Length < 12 || code.Length > 13) return false;

        var digits = code.Select(c => int.Parse(c.ToString())).Reverse().Skip(1).ToArray();
        var checkBit = int.Parse(code.Last().ToString());

        var evenIndexDigits = digits.Where((_, i) => i % 2 == 0).Sum();
        var oddIndexDigits = digits.Where((_, i) => i % 2 != 0).Sum();
        
        var parity = (evenIndexDigits * 3 + oddIndexDigits) % 10;
        if (parity != 0) parity = 10 - parity;
        return checkBit == parity;
    }
}