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;
}