ASP.net Response.Redirect on FileSystemWatcher event

Question

I don't manage to use a Response.Redirect (or Response.Write) on a new FileSystemWatcher event.

protected void Page_Load(object sender, EventArgs e)
{
   RunFolderListener();
}

private void RunFolderListener()
{
        FileSystemWatcher fsw = new FileSystemWatcher();
        fsw.Path = Server.MapPath("~/uploaded_images/");
        fsw.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite |
                           NotifyFilters.DirectoryName | NotifyFilters.FileName;
        fsw.Changed += new FileSystemEventHandler(OnChanged);
        fsw.EnableRaisingEvents = true;
}

    private void OnChanged(object source, FileSystemEventArgs e)
    {
        Response.Redirect(....); //not working
        Response.Write (....);//not working

    }

Answer 1

I'm afraid that's not going to work -- your folder watcher event isn't part of the page life cycle -- you would normally have some other thread or task to run to handle the FSW, then persist your state somewhere. Then you can say

protected void Page_Load(object sender, EventArgs e)
{
   if (myState.FileIngested) {
      Response.Redirect(....);
      Response.End();
   }
}

Hope that helps

Answer 2

What you want is impossible. The FileSystemWatcher events are handled by a different thread than the page thread. To illustrate this:

Page thread: |-Page_Load----...----Page_Unload-|
                 |-Start FSW
FSW thread :     |-----------------------------...--------FSW.OnChanged(...)---|

Somewhere inside Page_Load the FileSystemWatcher is started. The page lifecycle ends with a Page_UnLoad and the thread that ran the page lifecycle is returned to the thread pool. And more importantly, the page is rendered and finished and nothing can be changed anymore.

At some later moment in time, maybe one second, maybe ten minutes, the FileSystemWatcher generates an event. The page has been rendered by the browser a long time ago, so what do you think should happen when you call Response.Redirect? Since the response was already sent to the browser, nothing will happen.

Besides, this will eventually create memory problems. Each time you request this page, a new FileSystemWatcher is created. The watcher has to maintain a reference to the page because it has to call the event handler when an event occurs. Both the page and the watcher can therefore not be garbage-collected. So each time you request this page, you instantiate the page and a watcher that occupy a small amount of memory that is never reclaimed.

Answer 3

As the other's have mentioned, what you're looking to do is not possible. However, you can create a similar effect by:

1. Create a new FileSystemWatcher within your Global.asax's Application_Start event.
2. For the FileSystemWatcher OnChange event, have it update a Cache variable (ie Cache["UploadedImagesChanged"] with an object containing the info you need (ie LastChangeDateTime).
3. On your ASPX page, add a hidden value to hold a timestamp for the data within the page (ie HiddenField_LastUpdated.Value=DateTime.Now.ToString()).
4. Within your Page_Load, compare the current HiddenField_LastUpdated with the Cache["UploadedImagesChanged"] LastChangeDateTime. If the Cached date is newer, Response.Redirect the user.

You can also consider adding an asp:Timer to the page and some update panels if you want the page to poll the server to check for updates.