i want to zip memorystream but the size of zip file is incorrect my xml file size is 0

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C#5.0

I used this practice but I have a problem.
my file is very small 22 byte but it must be 56KB.
can anyone help me? I use code below:

I want save a table as xml and zip it and download it .I don't want save file on server and then download it

MemoryStream xmlfileStream = new MemoryStream();
xmlfileStream.Seek(0, SeekOrigin.Begin);

dsk.WriteXml(xmlfileStream);//dsk <--- this id a dataset
MemoryStream ms = new MemoryStream();
ms.Seek(0, SeekOrigin.Begin);
using (Ionic.Zip.ZipFile zipFile = new Ionic.Zip.ZipFile())
     {
           zipFile.AddEntry(ketab.Nam+".xml", xmlfileStream);
           xmlfileStream.Flush();
           zipFile.Save(ms);
     }