代码如下，res 赋不了值，为什么？

private HttpWebResponse GetData(string url, string agent = null, string prod_id = null, string oauth_consumerKey = "C16207CBF5444A5BB2499189D1E526D5", string oauth_consumerSecret = "8CC0EAFFED534E85AA4B7CE63784D4E2")

{

var oAuth = new CommonLib.OAuthBase();

var oauth_nonce = oAuth.GenerateNonce();

var oauth_timeStamp = oAuth.GenerateTimeStamp();

Uri uri = new Uri(url);

var authheader = oAuth.GenerateAuthorizationHeader(

uri,

oauth_consumerKey,

oauth_consumerSecret,

"GET",

oauth_timeStamp,

oauth_nonce

);

var req = WebRequest.Create(uri);

req.Headers[HttpRequestHeader.Authorization] = authheader;

if ("test3283".Equals(prod_id))

{

((HttpWebRequest)req).AddRange(30,100);

}

if (agent != null)

{

((HttpWebRequest)req).UserAgent = agent;

}

try

{

var res = req.GetResponse() as HttpWebResponse;

return res;

}

catch (WebException ex)

{

var res = ex.Response as HttpWebResponse;

return res;

}

}