Question

VB.NET Console App with FileSystemWatcher

Asked by: jacksonm1234

Hi everyone,

I have a console app that I'm using to monitor a folder using the filesystemwatcher class. When a new file is created in this folder, it should load a form, take a picture from the webcam, copy it to the clipboard,  and save the file to the HD. Here's the problem, if I load the form straight from my Main method (i.e. without using the filesystemwatcher events), it works perfectly. If I use the filesystemwatcher events, however, I get this error when trying to use the clipboard:

"Current thread must be set to single thread apartment (STA) mode before OLE calls can be made. Ensure that your Main function has STAThreadAttribute marked on it."

After some messing around with the ManagedThreadId property of the CurrentThread, I'm able to see that the thread ID of Main is 9 (or whatever), and the filesystemwatcher events and the form is on thread ID 10.  I'm guessing that I need to get the form to load from thread ID 9 in order for this to work properly.

I'm not good with delegates, but I've been trying everything I can think of, and I can't seem to get back to thread 9! Help!

Private Delegate Sub ShowForm(ByVal motion As Boolean)
Sub Main()
        MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        Console.Title = "Watcher"
        Dim watcher As New FileSystemWatcher("C:\Users\Mitch\Desktop\My Documents")
        watcher.NotifyFilter = NotifyFilters.FileName
        AddHandler watcher.Created, AddressOf FolderEvent
        watcher.EnableRaisingEvents = True
        While Console.ReadLine <> "exit" 
        End While
    End Sub  
'Here's where I tried using delegates to invoke the form
Private Sub FolderEvent(ByVal source As Object, ByVal e As FileSystemEventArgs)         
MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        If e.ChangeType = WatcherChangeTypes.Created Then
            If e.Name = "capture.txt" Then
                Dim sForm As New ShowForm(AddressOf ShowCam) 
                sForm.Invoke(False)
                'Dim wcp As New WebCamPreview(False)
                'wcp.ShowDialog()
.
.
.  
Public Sub ShowCam(ByVal motion As Boolean)
        MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        Dim wcp As New WebCamPreview(motion)
        wcp.ShowDialog()
    End Sub

                                  
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Asked On
2009-11-07 at 07:20:47ID24880469
Tags

Vb.net

,

console

,

delegates

,

threading

,

threads

,

filesystemwatcher

Topics

Microsoft Visual Basic.Net

,

.NET Framework 2.x

,

Visual Basic Programming

Participating Experts
1
Points
500
Comments
7

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Answers

 

by: x77Posted on 2009-11-07 at 08:36:53ID: 25766904

I use  Form1 with a buton to exit the application.
as delegate, use  
       BeginInvoke(New MethodInvoker(AddressOf ShowCam))

This use the same Message loop as form1.

Imports System.IO
Public Class Form1
Public Shared instance As Form1
 
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Me.Text = "Watcher"
        instance = Me
        Dim watcher As New FileSystemWatcher("C:\Documents and Settings\Leonardo\Escritorio")
        watcher.NotifyFilter = NotifyFilters.FileName
        AddHandler watcher.Created, AddressOf FolderEvent
        watcher.EnableRaisingEvents = True
        WindowState = FormWindowState.Minimized
End Sub
Private Filename As String
Private Sub FolderEvent(ByVal source As Object, ByVal e As FileSystemEventArgs)
        If e.ChangeType = WatcherChangeTypes.Created Then
            Filename = e.Name
            BeginInvoke(New MethodInvoker(AddressOf ShowCam))
        End If
End Sub
Public Sub ShowCam()
  Dim sform As New Form2
  sform.Text = Filename
  sform.Show(Me)
End Sub
End Class
                                              
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by: jacksonm1234Posted on 2009-11-07 at 09:09:21ID: 25767094

That makes sense, but is there any way to do this in a console application, where the filesystemwatcher is not in a form?

 

by: x77Posted on 2009-11-07 at 09:20:54ID: 25767139

Yes, replace invoke to BeginInvoke in your Sample.
I test it in Windows an put the sample.
The problem is that you use sync call and requires async.

 

by: jacksonm1234Posted on 2009-11-07 at 09:46:42ID: 25767217

I tried changing it to BeginInvoke, but I get the same error. Also, with that change, I ahve three separate threads instead of 2.  I would like to open the form from the original thread that Main is running on. Right now, its being called from the thread that the FileSystemWatcher is on.

 

by: x77Posted on 2009-11-07 at 10:21:55ID: 25767379

To use the same thread, you do no need invoke.
The simple mode is :

   Declare a variable bFindFile = False
   Replace the While loop to Wait for bFindFile changes.
   When bFindFile, then show the Form and reset bFindFile.
   Alter the variable bFindFile on FolderEvent when the File condition applies.

  You nedd to change the Console.ReadLine  to a function for inspect the keyboard and not blocking or create a thread to readline, then end the application.

  Note that the process, now is waiting by Console.Readline, then you can not use it to show the form.

 

by: x77Posted on 2009-11-07 at 10:34:18ID: 25767450

I Test your original with begininvoke and it works.
it run a process - ReadLine.
Next, when file created a new Trhead. This Thread is from S.O. call, you can terminate the function, you can not show the form in it.
Then You run a new thread to create the form and return (Return Control to FileSystemWatcher).
In the third thread, yo can show the form.

Imports System.IO
Module Module1
 
Private Delegate Sub ShowForm(ByVal motion As Boolean)
Sub Main()
        MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        Console.Title = "Watcher"
        Dim watcher As New FileSystemWatcher("C:\Documents and Settings\Leonardo\Escritorio")
        watcher.NotifyFilter = NotifyFilters.FileName
        AddHandler watcher.Created, AddressOf FolderEvent
        watcher.EnableRaisingEvents = True
        While Console.ReadLine <> "exit"
        End While
    End Sub
'Here's where I tried using delegates to invoke the form
Private Sub FolderEvent(ByVal source As Object, ByVal e As FileSystemEventArgs)
MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        If e.ChangeType = WatcherChangeTypes.Created Then
            'If e.Name = "capture.txt" Then
                Dim sForm As New ShowForm(AddressOf ShowCam)
                sForm.BeginInvoke(Nothing, Nothing, Nothing)
            'End If
        End If
End Sub
Public Sub ShowCam(ByVal motion As Boolean)
        MsgBox(System.Threading.Thread.CurrentThread.ManagedThreadId)
        Dim wcp As New WebCamPreview()
        wcp.ShowDialog()
    End Sub
End Module
                                              
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by: jacksonm1234Posted on 2009-11-07 at 14:28:39ID: 31651390

I ended up converting everything to a form instead of a console app.

20120131-EE-VQP-002

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