post到https的一个小坑

///zhh
private string Post(string url, string json)
        {
            System.Net.HttpWebRequest webRequest = System.Net.WebRequest.Create(url) as System.Net.HttpWebRequest;
            System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls;//关键修正
            System.Net.ServicePointManager.ServerCertificateValidationCallback += new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);

            webRequest.Method = "POST";
            webRequest.ContentType = "application/json;charset=UTF-8";
            webRequest.ServicePoint.Expect100Continue = false;
            webRequest.ProtocolVersion = System.Net.HttpVersion.Version11;//关键修正
            webRequest.Timeout = 20000;
            webRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)";
            webRequest.KeepAlive = false;
            string responseData = null;
            try
            {
                using (Stream requestStream = webRequest.GetRequestStream())
                {
                    byte[] data = System.Text.Encoding.ASCII.GetBytes(json);
                    requestStream.Write(data, 0, data.Length);
                }
                System.Net.HttpWebResponse response = webRequest.GetResponse() as System.Net.HttpWebResponse;
                using (Stream responseStrem = response.GetResponseStream())
                {
                    StreamReader responseReader = new StreamReader(responseStrem, System.Text.Encoding.UTF8);
                    responseData = responseReader.ReadToEnd();
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
           
            return responseData;
        }
        private bool CheckValidationResult(object sp, System.Security.Cryptography.X509Certificates.X509Certificate cert, System.Security.Cryptography.X509Certificates.X509Chain req, System.Net.Security.SslPolicyErrors sslError)
        {
            return true;
        }