how zip the xml and download

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ASP.NET

C#5.0

I have a dataset .I want save it as xml and zip it for user download it .
I use code below.
the xml file saves but the save dialog doesn't appear .how can I zip it and show save dialog to user

 datasetKetab dsk = new datasetKetab();
Ketab ketab = (from k in jle.Ketabs where k.KetabID == ketabid select k).Single();
                    dsk.Ketab.AddKetabRow(ketab.Nam, ketab.TedadeSafhe.Value, ketab.TedadeSoal.Value, ketab.Aks);
dsk.WriteXml(@"C:\Users\moslem\Desktop\a.xml");

ok it works. inested lonic I used system.io.compression.zipfile
but I get an error:"the process cannot access file because "zip file name" the file is using by another process"
xml and zip file is created in that folder but file will not download . why?

string directoryname=MapPath("path...");
                    string xmlfile = MapPath("path..." + ketab.Nam + ".xml");
                    dsk.WriteXml(xmlfile);
                    Response.ClearContent();
                    Response.ClearHeaders();
                                       Response.ContentType = "application/zip";
                    Response.AppendHeader("content-disposition", "attachment; filename=Report.zip");
                    string zipfile=MapPath("path..." + ketab.Nam+".zip");

                    ZipFile.CreateFromDirectory(directoryname, zipfile);//error is in this line
                    //File.Delete(xmlfile);
                    Response.WriteFile(zipfile);