C#で7DRの計算

/// 

/// 7DR　計算
/// NW-7
/// 
/// /// 
public static string Get7DR(string Value)
{
    bool result = Regex.IsMatch(Value, @"^[0-9]+$");
    if (!result)
    {
        return null;
    }

    string value = (ulong.Parse(Value) % 7).ToString();

    return value;
}