arrays - VB.NET Count number of files to copy with WebClient -

i'm trying write code copy files folder folder progress bar , label count files, copying 0 files of 10 files. code not working , throws , error.

collection modified; enumeration operation may not execute.

this code:

imports system.io imports system.net public class form1  dim filestocopy new arraylist() protected overrides sub onload(byval e eventargs)     mybase.onload(e)     copybtn.text = "copy file"     copybtn.parent = me     progbar.left = copybtn.right end sub dim withevents copybtn new button dim progbar new progressbar dim withevents filecopier new webclient public sub check(byval sender object, byval e eventargs) handles copybtn.click      dim src string = "d:\test"     dim dest string = "d:\test2"      each dir string in system.io.directory.getfiles(src)         dim dirinfo new system.io.directoryinfo(dir)         if not system.io.file.exists(dest & "\" & dirinfo.name)             filestocopy.add(dirinfo.name)         end if     next      if filestocopy.count > 0         if msgbox("there new files found. want sync now?", msgboxstyle.question + msgboxstyle.yesno, "confirm") = msgboxresult.yes             = 0 filestocopy.count - 1                 dim filecopier webclient = new webclient                 addhandler filecopier.downloadprogresschanged, addressof filecopier_downloadprogresschanged                 addhandler filecopier.downloadfilecompleted, addressof filecopier_downloadfilecompleted                  copybtn.enabled = false                 progbar.parent = me                 filecopier.downloadfileasync(new uri(src & "\" & filestocopy(i)), dest & "\" & filestocopy(i))              next         end if     else         msgbox("no new files copied")     end if end sub 

this code tracks number of files copying.

private sub filecopier_downloadprogresschanged(byval sender object, byval e downloadprogresschangedeventargs) handles filecopier.downloadprogresschanged     progbar.value = e.progresspercentage      each file in filestocopy         total += filestocopy.add(file)     next      label2.text = total \ filestocopy.count end sub end class 

add here instead in downloadprogresschanged

   = 0 filestocopy.count - 1             dim filecopier webclient = new webclient             addhandler filecopier.downloadprogresschanged, addressof filecopier_downloadprogresschanged             addhandler filecopier.downloadfilecompleted, addressof filecopier_downloadfilecompleted          copybtn.enabled = false         progbar.parent = me         filecopier.downloadfileasync(new uri(src & "\" & filestocopy(i)), dest & "\" & filestocopy(i))         label2.text = + 1 & "/" & filestocopy.count 'put here instead      next