Unable to append RichTextBox text in Cross-Thread operation VB.NET - vb.net

Question

I'am coding a Multi-Threaded TCP echo server, but the problem is when i call UpdateMessage from ClientHandler class, the RichTextBox's text is not appending.
This is the Form1 class that containts the RichTextBox1
Public Class Form1
Const PORT As Integer = 1234 'The port number on which the server will listen for connection.
Dim ServerSocket As New TcpListener(PORT) 'The Server Socket that will listen for connections on specified port number.
Dim Link As TcpClient 'The Socket that will handle the client.
Dim NumberOfClients As Integer = 0 'The total number of clients connected to the server.
Dim myThread As Thread 'The thread on which the server will handle the client
Dim sc As SynchronizationContext = SynchronizationContext.Current
Public Clienthandler As ClientHandler
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles btnStart.Click
ServerSocket.Start()
' UpdateMessage("Server started..")
myThread = New Thread(AddressOf AcceptClients) 'Handle the client in a different thread
myThread.Start()
btnStart.Text = "Started!"
btnStart.Enabled = False
End Sub
Private Sub AcceptClients()
'Keep accepting and handling the clients.
While (True)
Link = ServerSocket.AcceptTcpClient() 'Accept the client and let the Socket deal with it.
NumberOfClients = NumberOfClients + 1 'update the total number of clients
Dim ClientAddress As String = Link.Client.RemoteEndPoint.ToString
' UpdateMessage("Client number " & NumberOfClients & " connected from " & ClientAddress)
Clienthandler = New ClientHandler
Clienthandler.StartClientThread(Link, NumberOfClients)
Me.sc.Post(AddressOf Clienthandler.UpdateMessage, "Client number " & NumberOfClients & " connected from " & ClientAddress)
End While
'close the sockets
Link.Close() 'close the Socket.
ServerSocket.Stop() 'Stop the Server Socket.
End Sub
End Class
And here is the ClientHandler class:
Public Class ClientHandler
Private Link As TcpClient
Private ClientNumber As Integer
Dim NumberOfMessages As Integer = 0 'The number of messages that client has sent.
Dim Stream As NetworkStream 'The stream being used for sending/receiving data over Socket.
Dim scHandler As SynchronizationContext = SynchronizationContext.Current
Public Sub StartClientThread(Link As TcpClient, ClientNumber As Integer)
'Initialize the class variables with the arguments passed in StartClient
Me.Link = Link
Me.ClientNumber = ClientNumber
'Start the thread
Dim ClientThread As New Thread(AddressOf Chat)
ClientThread.Start()
UpdateMessage("Thread for client " & ClientNumber & " started!")
End Sub
Public Sub UpdateMessage2(Message As String)
Me.scHandler.Post(AddressOf UpdateMessage2, Message)
End Sub
Public Sub UpdateMessage(Message As String)
Message = vbNewLine & Trim(Message)
If Message.Length < 1 Then
Message = "empty"
End If
MessageBox.Show("Updating message!" & vbNewLine & Message)
Form1.RichTextBox1.AppendText(Message)
End Sub
The same problem still exits, the RichTextBox is being updated from UI Thread only.

Answer 1

To be a bit more specific than Hans Passant, using the form name where an object is expected makes use of the default instance of the form, which is an instance that the system creates and manages for you. The default instance is thread-specific though, which means that there is one default instance per thread. If you use the default instance in a secondary thread then you are using a different object to the one that you displayed on the UI thread. You might read a bit more about default instances here:
http://jmcilhinney.blogspot.com.au/2009/07/vbnet-default-form-instances.html
You need to somehow marshal a method call back to the UI thread and then update the existing form instance on that thread. There are a couple of ways that can be done:
Have access to the existing form object and call its Invoke method.
Use the SynchronizationContext class to marshal the method call to the UI thread and then use the default instance.

Answer 2

Class myClass
Friend TB As TextBox
Class myThread
' ...
End Class
End Class
Class Form1
Overrides Sub OnLoad
_tcpClient = New myClass
Controls.Add(_tcpClient.TB)
_tcpClient.doSomething()
End Sub
End Class
like that, as per my comments.