' m_messageAvailable As Long (IntPtr то бишь)

    If messageAvailable Then _
     .m_param.m_messageAvailable = FARPROC(AddressOf OpalMessageAvailableFunction)

Public Function FARPROC(ByVal pfn As Long) As Long
  
  'A dummy procedure that receives and returns
  'the return value of the AddressOf operator.
 
  'Obtain and set the address of the callback
  'This workaround is needed as you can't assign
  'AddressOf directly to a member of a user-
  'defined type, but you can assign it to another
  'long and use that (as returned here)
  'TextBoxAdd Form1.TextGetMes, CStr(pfn)
 
  FARPROC = pfn

End Function

Public Function OpalMessageAvailableFunction() As Long
  OpalMessageAvailableFunction = 1
  FormMain.TimerGetMes.Enabled = True
End Function

Private Sub TimerGetMes_Timer()
  Dim MesPtr As Long
  TimerGetMes.Enabled = False
  Do
    MesPtr = OpalGetMessage(hOpal, 0)
    If MesPtr <> 0 Then
      HandleMessages MesPtr
    Else
      Exit Do
    End If
  Loop
End Sub

    /// <summary>
    /// If non-null then this function is called before
    /// the message is queued for return in the
    /// GetMessage(). See the
    /// OpalMessageAvailableFunction for more details.
    /// </summary>
    public OpalMessageAvailableFunction MessageAvailable
    {
      get { return (OpalMessageAvailableFunction)Marshal.GetDelegateForFunctionPointer(m_opalParamGeneral.m_messageAvailable, typeof(OpalMessageAvailableFunction)); }
      set { m_opalParamGeneral.m_messageAvailable = Marshal.GetFunctionPointerForDelegate(value); }
    }

    public delegate int OpalMessageAvailableFunction([MarshalAs(UnmanagedType.LPStruct)] Opal_API.OpalMessage message); /**< Message that has become available. */   

  Public Delegate Function OpalMessageAvailableFunction() As Integer ' Message that has become available.

  Public MessageAvailableFunction As OpalMessageAvailableFunction = _
   New OpalMessageAvailableFunction(AddressOf MyMessageAvailable) 'чтоб мусорщик не убил

        .m_param.m_messageAvailable = Marshal.GetFunctionPointerForDelegate(MessageAvailableFunction)

  Public Function MyMessageAvailable() As Integer
    Form1.TimerGetMes.Enabled = True
    Debug.Print("message")
    Return 1
  End Function

  Public Form1 As frm1

  Sub main()
...
    Form1 = New frm1
    Application.Run(Form1)
  End Sub

  Private Sub TimerGetMes_Tick(sender As Object, e As EventArgs) Handles TimerGetMes.Tick
    TimerGetMes.Enabled = False
    Dim MesPtr As IntPtr
    Do
      MesPtr = OpalGetMessage(hOpal, 0)
      If MesPtr <> IntPtr.Zero Then
        'HandleMessages MesPtr
        Debug.Print(MesPtr)
        OpalFreeMessage(MesPtr)
      Else
        Exit Do
      End If
    Loop
    Debug.Print("Exit")
End Sub

message
message
message
message
message
'очевидно истекла 1мс от прихода первого "message" и сработал тик таймера, выключаем таймер из-под себя и им же читаем накопленные сообщения
130428280
130468296
130470496
130527000
130469496
Exit 'сообщения закончились, тик таймера отработал
'приходят новые сообщения (может через несколько секунд, может через полминуты)
message
message
message
message
' и почему-то тик таймера больше не вызывается, хотя мы "включаем" его при каждом message

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    MsgBox(TimerGetMes.Enabled)
  End Sub

Private Sub TimerGetMes_Tick(sender As Object, e As EventArgs) Handles TimerGetMes.Tick

public delegate int OpalMessageAvailableFunction([MarshalAs(UnmanagedType.LPStruct)] Opal_API.OpalMessage message);

Public Delegate Function OpalMessageAvailableFunction() As Integer

  Public Function MyMessageAvailable() As Integer
    Debug.Print("message")
    Form1.TimerGetMes.Enabled = True
    Return 1
  End Function

  Public Function MyMessageAvailable() As Integer
     Debug.Print("message")
     SetTimer(Form1.Handle, New IntPtr(1), 1, Nothing)
    'Form1.StartTimerFromMessageAvailable()
    Return 1
  End Function

  Public Function MyMessageAvailable() As Integer
    PostMessage(hForm1, WM_TIMER, 1001, IntPtr.Zero)
    Return 1
  End Function

 Public hForm1 As IntPtr

  Private Sub frm1_Load(sender As Object, e As EventArgs) Handles Me.Load
...
    hForm1 = Me.Handle
  End Sub

  Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
    'Debug.WriteLine(m.ToString())
    Select Case m.Msg
      Case WM_TIMER
        If m.WParam = New IntPtr(1001) Then
          Dim MesPtr As IntPtr
          Do
            MesPtr = OpalGetMessage(hOpal, 0)
            If MesPtr <> IntPtr.Zero Then
              'HandleMessages MesPtr
              Debug.Print(MesPtr.ToString)
              OpalFreeMessage(MesPtr)
            Else
              Exit Do
            End If
          Loop
        End If
    End Select

    ' Forward message to base WndProc.
    MyBase.WndProc(m)
  End Sub