使用HttpWebRequest请求Https网站时，程序抛出错误：System.Net.WebException:“请求被中止: 未能创建 SSL/TLS 安全通道。”

具体代码如下：

            if (en == null) en = Encoding.UTF8;
            String html;

            if (url.IndexOf("https") > -1)
            {
                ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
                ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls;
            }
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
            request.Timeout = 600000;

          
            //这里报错：System.Net.WebException:“请求被中止: 未能创建 SSL/TLS 安全通道。”
            WebResponse response = request.GetResponse();

            Stream dataStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(dataStream, en);
            String responseFromServer = reader.ReadToEnd();
            html = responseFromServer;
            Encoding en2 = GetEncoding(response.ContentType);//responseFromServer
            reader.Close();
            response.Close();