ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated because the current thread is not in a single-threaded apartment

Question

I am getting this error when i try to show a form with an webbrowser in it.

ActiveX control \'8856f961-340a-11d0-a96b-00c04fd705a2\' cannot be instantiated becau

Answer 1

The problem happens because a UI component is being created on a non-UI thread. Here is a simplified version of code which will break with this error message.

Imports System.ComponentModel

Public Class Form1
    Private _ThreadedProcesss As ThreadedProcess
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        _ThreadedProcesss = New ThreadedProcess()
        _ThreadedProcesss.Start()
    End Sub

    Public Class ThreadedProcess
        Private _thread As System.Threading.Thread

        Public Sub Start()
            _thread = New System.Threading.Thread(AddressOf ThreadEntryPoint)
            _thread.Name = "Testing thread"
            _thread.Start()
        End Sub

        Private Sub ThreadEntryPoint(status As Object)
            Dim wb As New WebBrowser()
            wb.Dock = DockStyle.Fill
            wb.Visible = True
            wb.DocumentText = "Hello world"
        End Sub
    End Class
End Class

To correct the problem, you just need to get back to the UI thread before creating the control.

Imports System.ComponentModel

Public Class Form1
    Private _ThreadedProcesss As ThreadedProcess
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        _ThreadedProcesss = New ThreadedProcess(New EventHandler(AddressOf ThreadedProcess_Create))
        _ThreadedProcesss.Start()
    End Sub

    Private Sub ThreadedProcess_Create(sender As Object, e As System.EventArgs)
        If (Me.InvokeRequired) Then
            Me.Invoke(New EventHandler(AddressOf ThreadedProcess_Create), {sender, e})
        Else
            Dim wb As New WebBrowser()
            Me.Controls.Add(wb)
            wb.Dock = DockStyle.Fill
            wb.Visible = True
            wb.DocumentText = "Hello world"
        End If
    End Sub

    Public Class ThreadedProcess
        Private _thread As System.Threading.Thread
        Private _callback As EventHandler

        Public Sub New(callback As EventHandler)
            _callback = callback
        End Sub

        Public Sub Start()
            _thread = New System.Threading.Thread(AddressOf ThreadEntryPoint)
            _thread.Name = "Testing thread"
            _thread.Start()
        End Sub

        Private Sub ThreadEntryPoint(status As Object)
            _callback.Invoke(Me, EventArgs.Empty)
        End Sub
    End Class
End Class

So for your example I believe the code would be:

 Class Server
    Private _callback as EventHandler
    Public Sub Main(callback as EventHandler)
        _callback = callback
        Dim aTcpMessaging As IMessagingSystemFactory = New TcpMessagingSystemFactory()
        Dim anInputChannel As IInputChannel = aTcpMessaging.CreateInputChannel(theIPforLocal & ":" & thePort)
        Dim aStringMessagesFactory As IStringMessagesFactory = New StringMessagesFactory()
        Dim aStringMessageReceiver As IStringMessageReceiver = aStringMessagesFactory.CreateStringMessageReceiver()
        AddHandler aStringMessageReceiver.MessageReceived, AddressOf StringMessageReceived

        aStringMessageReceiver.AttachInputChannel(anInputChannel)
    End Sub

    Private Sub StringMessageReceived()
        _callback.Invoke(Me, EventArgs.Empty)
    End Sub
  End Class

  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
      Dim RelState As Integer = 0
      Call frmMain.Server.Main(new EventHandler(AddressOf Server_MessageReceived))   '<-- the error now
      lblVer.Text = "V.7"
      Pid = 0
  End Sub
  Private Sub Server_MessageReceived(sender as Object, e As EventArgs)
      If (Me.InvokeRequired) Then
          Me.Invoke(New EventHandler(AddressOf Server_MessageReceived), {sender, e})
      Else
          Call New frmMM().Show()
      End If
  End Sub
End Class