Make Zip File during download

Using zip As New ZipFile()

            For i As Integer = 0 To lbFilesDetails.Items.Count - 1
                If lbFilesDetails.Items(i).Selected = True Then
                    zip.AddFile(lbFilesDetails.Items(i).Value.ToString())
                End If
            Next

            If zip.Count > 0 Then
                Response.Clear()
                Response.BufferOutput = False
                Dim zipName As String = [String].Format("Zip_{0}.zip", txtXXXNum.Text)
                Response.ContentType = "application/zip"
                Response.AddHeader("content-disposition", "attachment; filename=" + zipName)
                zip.Save(Response.OutputStream)
                Response.[End]()
            Else
                ScriptManager.RegisterStartupScript(Me, [GetType](), "showalert", "alert('No File(s) Selected or Exists.');", True)
            End If
        End Using