Here is my question in short: How do I use the BackGroundWorker (or InvokeRequired method) to make thread-safe calls to append text to a text box?
Here is my question in with much detail and background: I've been working on a program that copies file from one location to another for backup purposes. I set an option that will save a file when the file is modified using the FileSysteWatcher. Here is the code:
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.Load Dim directoryPath As String = Path.GetDirectoryName(TextBox1.Text) Dim varFileSystemWatcher As New FileSystemWatcher() varFileSystemWatcher.Path = directoryPath varFileSystemWatcher.NotifyFilter = (NotifyFilters.LastWrite) varFileSystemWatcher.Filter = Path.GetFileName(TextBox1.Text) AddHandler varFileSystemWatcher.Changed, AddressOf OnChanged varFileSystemWatcher.EnableRaisingEvents = True End Sub Private Sub OnChanged(source As Object, ByVal e As FileSystemEventArgs) My.Computer.FileSystem.CopyFile(e.FullPath, TextBox2.Text & "\" & e.Name, True) TextBox3.ApendText("[New Text]") ' This line causes an error End Sub
The code works fine except for updating the text in textbox3. I need the textbox to update when the file specified is modified. This is important so that the user can know the program is working and to have a complete log of the programs operations.
The error that is caused is:
Cross-thread operation not valid: Control 'TextBox3' accessed from a thread other than the thread it was created on.
I'm assuming "AddHandler varFileSystemWatcher.Changed, AddressOf OnChanged" creates a new thread. Also, updating "textbox3" within the "OnChange Sub" (in the way I did) is not a thread-safe manner. So I did some research and found this article:
How to: Make Thread-Safe Calls to Windows Forms Controls
The article explains that one can use InvokeRequired or a BackgroundWorker to make thread-safe calls. I would like to use the BackgroundWorker to make a thread-safe call (if InvokeRequired is more efficient then I'll use that), but this portion, of the example provied, leaves me confused:
' Do I need these imports to use the BackgroundWorker or InvokeRequired? Imports System Imports System.ComponentModel Imports System.Threading Imports System.Windows.Forms Public Class Form1 Inherits Form ' Do I need this for what I am trying to do? ' This delegate enables asynchronous calls for setting ' the text property on a TextBox control. Delegate Sub SetTextCallback([text] As String) ' This thread is used to demonstrate both thread-safe and ' unsafe ways to call a Windows Forms control. Private demoThread As Thread = Nothing ' This BackgroundWorker is used to demonstrate the ' preferred way of performing asynchronous operations. Private WithEvents backgroundWorker1 As BackgroundWorker Private textBox1 As TextBox Private WithEvents setTextUnsafeBtn As Button Private WithEvents setTextSafeBtn As Button Private WithEvents setTextBackgroundWorkerBtn As Button ' What is this part of the code for and do I need it? Private components As System.ComponentModel.IContainer = Nothing ' Again, What is this part of the code for and do I need it? Public Sub New() InitializeComponent() End Sub ' And again, What is this part of the code for and do I need it? Protected Overrides Sub Dispose(disposing As Boolean) If disposing AndAlso (components IsNot Nothing) Then components.Dispose() End If MyBase.Dispose(disposing) End Sub Many parts of the above code leave me confused. What does it do? What parts of this code do I need to use the BackgroundWorker? What parts for the InvokeRequired method? Again, How do I use the BackGroundWorker (or InvokeRequired method) to make thread-safe calls to append text to a text box? (It'd be great to have the code above explained, but all I really need is one example of how to update the text of a text box in a thread-safe manner.)