如题，详细代码如下： [图片] 提示：基础连接已经关闭: 发送时发生错误

如题，在使用统一下单接口时，提交数据时报，基础连接已经关闭: 发送时发生错误。这种情况是什么原因造成的呢？ 代码如下:             System.GC.Collect();//垃圾回收             ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;             var data = Encoding.GetEncoding("UTF-8").GetBytes(postStr);             Stream responseStream;             HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;             request.ServicePoint.ConnectionLimit = 300;             if (request == null)             {                 throw new ApplicationException(string.Format("Invalid url string: {0}", url));             }             // request.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";               request.ContentType = "application/x-www-form-urlencoded";             request.Method = "POST";             request.ContentLength = data.Length;             Stream requestStream = request.GetRequestStream();//报错点             requestStream.Write(data, 0, data.Length);             requestStream.Close();             try             {                 responseStream = request.GetResponse().GetResponseStream();             }             catch (Exception exception)             {                 throw exception;             }             string str = string.Empty;             using (StreamReader reader = new StreamReader(responseStream, Encoding.GetEncoding("UTF-8")))             {                 str = reader.ReadToEnd();             }             responseStream.Close();