Method 屬性設置為任何 HTTP 1.1 協議謂詞：GET、HEAD、POST、PUT、DELETE、TRACE 或 OPTIONS。

static void Main(string[] args)
{
    var url = "http://www.google.com/intl/en_ALL/images/srpr/logo1w.png";
    var len = GetHttpLength(url);
    Console.WriteLine("Url:{0}/r/nLength:{1}", url,len);
}
static long GetHttpLength(string url)
{
    var length = 0l;
    try
    {
        var req = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url));
        req.Method = "HEAD";
        req.Timeout = 5000;
        var res = (HttpWebResponse)req.GetResponse();
        if (res.StatusCode == HttpStatusCode.OK)
        {
            length =  res.ContentLength; 
        }
        res.Close();
        return length;
    }
    catch (WebException wex)
    {
        return 0;
    }
}