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();