function MustUseCache()
    {
        if ($this->cacheLifetime == 0)
        {
            return false;
        }
        if (!File::Exists($this->file))
        {
            return false;
        }
        $now = Date::Now();
        $lastMod = File::GetLastModified($this->file);
        if ($now->TimeStamp() - $lastMod->TimeStamp() < $this->cacheLifetime)
        {
            return true;
        }
        return false;
    }