/// <summary>  
        /// 创建GET方式的HTTP请求  
        /// </summary>  
        /// <param name="url">请求的URL</param>  
        /// <param name="timeout">请求的超时时间</param>  
        /// <param name="userAgent">请求的客户端浏览器信息，可以为空</param>  
        /// <param name="cookies">随同HTTP请求发送的Cookie信息，如果不需要身份验证可以为空</param>  
        /// <returns></returns>  
        public static HttpWebResponse CreateGetHttpResponse(string url,int? timeout, string userAgent,CookieCollection cookies)  
        {  
            if (string.IsNullOrEmpty(url))  
            {  
                throw new ArgumentNullException("url");  
            }  
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;  
            request.Method = "GET";  
            request.UserAgent = DefaultUserAgent;  
            if (!string.IsNullOrEmpty(userAgent))  
            {  
                request.UserAgent = userAgent;  
            }  
            if (timeout.HasValue)  
            {  
                request.Timeout = timeout.Value;  
            }  
            if (cookies != null)  
            {  
                request.CookieContainer = new CookieContainer();  
                request.CookieContainer.Add(cookies);  
            }  
            return request.GetResponse() as HttpWebResponse;  
        }