Question

I’m trying to write a simple program to monitor a folder for new files in VB.NET 2010, and am having some trouble.

Here’s a simplified version of what my program looks like:

Imports System.IO

Public Class Main
    Public fileWatcher As FileSystemWatcher

    Sub btnGo_Click(sender As System.Object, e As System.EventArgs) Handles btnGo.Click
        '//# initialize my FileSystemWatcher to monitor a particular directory for new files
        fileWatcher = New FileSystemWatcher()
        fileWatcher.Path = thisIsAValidPath.ToString()
        fileWatcher.NotifyFilter = NotifyFilters.FileName
        AddHandler fileWatcher.Created, AddressOf fileCreated
        fileWatcher.EnableRaisingEvents = True
    End Sub

    Private Sub fileCreated(sender As Object, e As FileSystemEventArgs)
        '//# program does not exit when I comment the line below out
        txtLatestAddedFilePath.Text = e.FullPath
        '//# e.FullPath is valid when I set a breakpoint here, but when I step into the next line, the program abruptly halts with no error code that I can see
    End Sub
End Class

As you can see, I have a button which will initialize a FileSystemWatcher when clicked. The initialization works, and when I place a new file in the monitored directory, the program reaches the fileCreated sub. I can even see that e.FullPath is set correctly. However, it exits abruptly right after that with no error code (none that I can see, anyways). If I comment everything in the fileCreated sub out, the program continues running as expected.

Any ideas as to why it’s dying on me? Any help would be greatly appreciated. I’m fairly new to VS/VB.NET, so maybe I’m just making a silly mistake. Thanks!

Answer 1

Could be a cross-thread operation exception.

Try this:

Private Sub fileCreated(sender As Object, e As FileSystemEventArgs)
  me.Invoke(New MethodInvoker(Function() txtLatestAddedFilePath.Text = e.FullPath))
End Sub

or (even better in your context), during fileWatcher initialization:

fileWatcher = New FileSystemWatcher()
fileWatcher.SynchronizingObject = me
[...]

---

Explanation:

http://www.blackwasp.co.uk/FileSystemWatcher.aspx (see Preventing Cross-Thread Operations)

Excerpt:

By default, when the FileSystemWatcher
object raises notification events, the
delegate calls are made on a thread
from the system thread pool. This will
generally not be the same thread as
that being used to control the form.
As the demonstration application will
require that the file changes be
logged within a visual element of the
form, using the allocated thread to
modify the list box contents would
result in a cross-threading operation
and an IllegalOperationException being
thrown.