C#でモジュラス11の計算

/// 

/// モジュラス11　計算
/// NW-7
/// 
/// /// 
public static string GetModulus11(string Value)
{
    bool result = Regex.IsMatch(Value, @"^[0-9]+$");
    if (!result || 6 > Value.Length)
    {
        return null;
    }

    string val = Value.Substring(0, 6);

    int checkDigit = 0;
    for (int i = 0; i < val.Length; i++)
    {
        checkDigit += int.Parse(val.Substring(i, 1)) * (7 - i);
    }
    checkDigit = checkDigit % 11;

    if (checkDigit == 0)
    {
        return "1";
    }
    else if (checkDigit == 1)
    {
        return "0";
    }
    else
    {
        return (11 - checkDigit).ToString();
    }
}