httpWebRequest请求错误,基础连接已经关闭: 连接被意外关闭

public string DoWebRequest(string url)
        {
            HttpWebResponse webResponse = null;
           HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);//根据URL来访问网址
            webRequest.Method = "POST";
            string responseStr = null;
            webRequest.Timeout = 50000; //设置超时时间
            webRequest.ContentType = "text/html; charset=gb2312";
            try
            {
                //尝试获得要请求的URL的返回消息
                webResponse = (HttpWebResponse)webRequest.GetResponse();
            }
            catch (WebException e)
            {
                //发生网络错误时,获取错误响应信息
                responseStr = "发生网络错误！请稍后再试";
            }
            catch (Exception e)
            {
                //发生异常时把错误信息当作错误信息返回
                responseStr = "发生错误：" + e.Message;
 
            }
            finally
            {
                if (webResponse != null)
                {
                    //获得网络响应流
                    using (StreamReader responseReader = new StreamReader(webResponse.GetResponseStream(), Encoding.GetEncoding("GB2312")))
                    {
                        responseStr = responseReader.ReadToEnd();//获得返回流中的内容
                    }
                    webResponse.Close();//关闭web响应流
                }
            }
            return responseStr;
        }