【解決方法】C#でcer証明書を使用してhttpwebrequest経由で応答を取得する方法は?

C# で cer 証明書を使用して HTTPwebRequest 経由で応答をフェッチしたい。 拡張子が .cer の証明書を信頼されたルート証明書ストアにインポートしました。 GetResponse メソッドで応答を取得しているときに、応答が null になっています

私が試したこと：

私のコードは以下の通りです：

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls
               | SecurityProtocolType.Tls11
               | SecurityProtocolType.Tls12
               | SecurityProtocolType.Ssl3;


                ServicePointManager.Expect100Continue = true;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(txtRequestURL.Text);
                System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
                request.Method = "POST";
                request.Accept = @"application/json";
                request.ContentType = @"application/json";
                Byte[] byteArray = encoding.GetBytes(txtReq.Text);
                request.ContentLength = byteArray.Length;
                if (chkRequestHeader.Checked)
                {

                    request.Headers.Add("X-API-KEY", txtXAPIKey.Text);
                }
               
               

                //request.Timeout = 1000 * Convert.ToInt32(txtResponseTimeOut.Text);

                  string certificatePath = @"D:\Cert.cer";
                   
                  request.ClientCertificates.Add(new X509Certificate(certificatePath));

                 txtLog.Text += "btnInvoke_Click()-Adding Certificates Process Completed with Certificate path:"+certificatePath + Environment.NewLine;
               

               using (Stream stm = request.GetRequestStream())

                {
                    
                    stm.Write(byteArray, 0, byteArray.Length);
                    
                }


                try
                {
                   
                    HttpWebResponse httpWebRes = (HttpWebResponse)request.GetResponse();
                  
                    txtHTTPStatusCode.Text = Convert.ToString(httpWebRes.StatusCode);
                    using (StreamReader responseReader = new StreamReader(httpWebRes.GetResponseStream()))
                    {

                        try
                        {
                           
                            txtResponse.Text = responseReader.ReadToEnd();
                            
                            
                        }
                        catch (Exception ex)
                        {

                            txtLog.Text += "btnInvoke_Click()-Fetching Response from Response Stream Failed with Exception: " + ex.Message
                               + Environment.NewLine + " with Inner Exception:" + ex.InnerException + Environment.NewLine + " at Stack Trace: " + ex.StackTrace + Environment.NewLine;
                        }
                      
                        
                        responseReader.Close();
                       

                    }