Convert multiple attachment to single zip in email

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C#3.5

Hi,

I was sending multiple attachement but as suggested by someone, I need to convert it into the single zip. Can anyone suggest what change I need to implement in given code.

public bool GetCertificatePDF(List<CPETSCertificate> objCPETSCertificate, string Name, string EmailId)
       {
           try
           {
               List<Attachment> lstAttachment = new List<Attachment>();
               int certificateCount = objCPETSCertificate.Count();
               string EnvironmentFrontEndUrl = System.Configuration.ConfigurationManager.AppSettings["EnvironmentFrontEndUrl"].ToString();
               for (int i = 0; i < certificateCount; i++)
               {
                   if (objCPETSCertificate[i].CourseName.Trim() != "" && objCPETSCertificate[i].CourseName.Trim() != null)
                   {
                       StreamReader stringHtml = System.IO.File.OpenText(System.Web.Hosting.HostingEnvironment.MapPath("~/assets/dictionary/lrn-cpets-certificate.html"));
                       string changeHtml = stringHtml.ReadToEnd();
                       if (certificateCount > 0 && certificateCount < 3)
                       {

                           changeHtml = changeHtml.Replace("##BG##", EnvironmentFrontEndUrl + "/assets/images/cpet/standard.jpg");
                       }
                       else if (certificateCount > 2 && certificateCount < 6)
                       {
                           changeHtml = changeHtml.Replace("##BG##", EnvironmentFrontEndUrl + "/assets/images/cpet/bronze.jpg");
                       }
                       else if (certificateCount > 6 && certificateCount < 10)
                       {
                           changeHtml = changeHtml.Replace("##BG##", EnvironmentFrontEndUrl + "/assets/images/cpet/silver.jpg");
                       }
                       else if (certificateCount == 10)
                       {
                           changeHtml = changeHtml.Replace("##BG##", EnvironmentFrontEndUrl + "/assets/images/cpet/gold.jpg");
                       }
                       changeHtml = changeHtml.Replace("##DictionaryListing##", Name.ToUpper());
                       changeHtml = changeHtml.Replace("##DictionaryListing2##", HttpUtility.UrlDecode(objCPETSCertificate[i].CourseName.ToUpper()));
                       if (objCPETSCertificate[i].CompletionDate != null)
                       {
                           changeHtml = changeHtml.Replace("##DictionaryListing3##", Convert.ToString(objCPETSCertificate[i].CompletionDate.ToString("dd-MMM-yyyy").Replace(" ", "")));
                       }
                       else
                       {
                           changeHtml = changeHtml.Replace("##DictionaryListing3##", string.Empty);
                       }
                       byte[] Certificate_PDF = GeneratePDFusingEvo(changeHtml, objCPETSCertificate[i].CourseName);
                       string CertificateName = (Name + "SiriusDecisions" + objCPETSCertificate[i].CourseName + objCPETSCertificate[i].CompletionDate.ToString("MM.dd.yyyy") + ".pdf").Replace(" ", "");
                       Attachment att = new Attachment(new MemoryStream(Certificate_PDF), CertificateName);
                       lstAttachment.Add(att);
                   }
               }
               bool flag = SendCPETSCertificate(Name, EmailId, lstAttachment);
               return flag;
           }
           catch (Exception ex)
           {
               Sitecore.Diagnostics.Log.Error(ex.Message, this);
               return false;
           }
       }

What I have tried:

As I am directly converting byte[] to attachment, I have not stored any physical file and I don't want to do it. Just need to convert List<attachment> to single zip