Update Text Box Properly when Cross-threading in Visual Basic (VS 2012 V11)

Question

• 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?

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Answer 1

Change:

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

To:

Private Sub OnChanged(source As Object, ByVal e As FileSystemEventArgs)
    My.Computer.FileSystem.CopyFile(e.FullPath, TextBox2.Text & "\" & e.Name, True)
    AppendTextBox(TextBox3, "[New Text]")
End Sub

Private Delegate Sub AppendTextBoxDelegate(ByVal TB As TextBox, ByVal txt As String)

Private Sub AppendTextBox(ByVal TB As TextBox, ByVal txt As String)
    If TB.InvokeRequired Then
        TB.Invoke(New AppendTextBoxDelegate(AddressOf AppendTextBox), New Object() {TB, txt})
    Else
        TB.AppendText(txt)
    End If
End Sub

Here's an updated, simplified version that uses an anonymous delegate:

Private Sub OnChanged(source As Object, ByVal e As FileSystemEventArgs)
    Dim filename As String = System.IO.Path.Combine(TextBox2.Text, e.Name)
    Try
        My.Computer.FileSystem.CopyFile(e.FullPath, filename, True)

        TextBox3.Invoke(Sub()
                            TextBox3.AppendText("[New Text]")
                        End Sub)

    Catch ex As Exception
        Dim msg As String = "Source: " & e.FullPath & Environment.NewLine &
            "Destination: " & filename & Environment.NewLine & Environment.NewLine &
            "Exception: " & ex.ToString()
        MessageBox.Show(msg, "Error Copying File")
    End Try
End Sub

Answer 2

You can also use SynchronizationContext. Be careful to get a reference to it from the constructor. There's a great article on this at CodeProject.com: http://www.codeproject.com/Articles/14265/The-NET-Framework-s-New-SynchronizationContext-Cla

Imports System.IO
Imports System.Threading

Public Class Form1
    Private m_SyncContext As System.Threading.SynchronizationContext
    Private m_DestinationPath As String

    Public Sub New()

        ' This call is required by the designer.
        InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.
        Me.m_SyncContext = System.Threading.SynchronizationContext.Current
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.m_DestinationPath = Me.TextBox2.Text
        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, Me.m_DestinationPath & "\" & e.Name, True)
        Me.m_SyncContext.Post(AddressOf UpdateTextBox, "[New Text]")
    End Sub

    Private Sub UpdateTextBox(param As Object)
        If (TypeOf (param) Is String) Then
            Me.TextBox3.AppendText(CStr(param))
        End If
    End Sub
End Class