底层连接关闭:发送时发生意外错误

我使用HttpWebRequest类异步如下代码(它只是windows应用程序)

    private void StartWebRequest(string url)
    {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            request.BeginGetResponse(new AsyncCallback(FinishWebRequest), request);

    }
    private void FinishWebRequest(IAsyncResult result)
    {
            HttpWebResponse response = (result.AsyncState as HttpWebRequest).EndGetResponse(result) as HttpWebResponse;
            Stream responseStream = response.GetResponseStream();
            int num = 100000;
            byte[] buffer = new byte[num];
            int offset = 0;
            while ((num2 = responseStream.Read(buffer, offset, 1000)) != 0)
            {
                offset += num2;
            }
            MemoryStream stream = new MemoryStream(buffer, 0, offset);
            Bitmap bitmap = (Bitmap)Image.FromStream(stream);
            bitmap.Save(@"z:'new.jpg");
            response.Close();
            responseStream.Close();
            stream.Close();
    }

有时会出现这个错误:
底层连接已关闭:发送时发生意外错误
有办法解决这个问题吗?
谢谢