<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)>
  Public Structure COPYDATASTRUCT_UNI
    Dim dwData As IntPtr ' additional 32 bit data to pass
    Dim cbData As Integer ' length in bytes of data pointed by lpData
    Dim lpData As String ' pointer to data to be passed to the receiving app. >
  End Structure

  Public Declare Unicode Function SendMessage_COPYDATASTRUCT_UNI Lib "user32" Alias "SendMessageW" _
   (ByVal hwnd As IntPtr, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As COPYDATASTRUCT_UNI) As Integer

  Private Sub ButtonSendMeUNIOrig_Click(sender As Object, e As EventArgs) Handles ButtonSendMeUNIOrig.Click
    Dim cds As New COPYDATASTRUCT_UNI
    cds.dwData = CType(3, IntPtr) 'идентифицирует тип передаваемых данных
    cds.cbData = System.Text.Encoding.Unicode.GetByteCount(TextBoxSendMeUNI.Text)
    cds.lpData = TextBoxSendMeUNI.Text

    Call SendMessage_COPYDATASTRUCT_UNI(Me.Handle, WM_COPYDATA, Me.Handle, cds)

  End Sub

  <StructLayout(LayoutKind.Sequential)>
  Public Structure COPYDATASTRUCT
    Dim dwData As IntPtr ' additional 32 bit data to pass
    Dim cbData As Integer ' length in bytes of data pointed by lpData
    Dim lpData As IntPtr ' pointer to data to be passed to the receiving app.
  End Structure

      Case WM_COPYDATA
        Dim the_str As String
        Dim cds As COPYDATASTRUCT = CType(Marshal.PtrToStructure(m.LParam, GetType(COPYDATASTRUCT)), COPYDATASTRUCT)
        Dim buffer(cds.cbData - 1) As Byte
        Marshal.Copy(cds.lpData, buffer, 0, cds.cbData)
        Using mstream As New IO.MemoryStream(buffer)
          Using reader As New IO.StreamReader(mstream, System.Text.Encoding.Unicode, True)
            the_str = reader.ReadToEnd()
          End Using
        End Using
        MsgBox(the_str & ";len=" & Strings.Len(the_str))
      Case Else
    End Select

Public Class Form1

  Private Delegate Sub MyDelegate(parameter As String)

  Public Sub DoIt(parameter As String)
    Me.Invoke(New MyDelegate(AddressOf ui), parameter)
  End Sub

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    DoIt("ha-ha")
  End Sub

  Private Sub ui(parameter As String)
    Do
      Me.Text = Now & " " & parameter
    Loop
  End Sub

End Class

Imports System.Threading

Public Class Form1

  Private myThread As Thread
  Private Delegate Sub MyDelegate(parameter As String)

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    myThread = New Thread(New ThreadStart(AddressOf ThreadFunction)) 'здесь запускаем поток
    myThread.Start()
  End Sub

  Private Sub Form1_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
    myThread.Abort() 'здесь останавливаем/убиваем поток, чтоб предотвратить возможные исключения
  End Sub

  Private Sub ThreadFunction() 'здесь делаем "долбежные дела", может циклиться, может спать, может ждать
    Do
      'Thread.Sleep(100)
      Me.Invoke(New MyDelegate(AddressOf ui), Now().ToString) 'стучимся в ui, передавая параметр(ы)=команда
    Loop
  End Sub

  Private Sub ui(parameter As String) 'здесь обрабатываем команду в потоке формы(полный доступ к ее интерфейсу)
    Me.Text = parameter
  End Sub

  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    MsgBox("Not blocked")
  End Sub
End Class

Public Class Form1
  Dim MailSlotThread As Thread
  Private Delegate Sub MailSlotDelegate(ByVal NewMessage As String)

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    If MakeMailSlot("\\.\mailslot\MailSlotMyName", True) Then
      MailSlotThread = New Thread(New ThreadStart(AddressOf MailSlotThreadFunction))
      MailSlotThread.Start()
    End If
  End Sub

  Private Sub MailSlotThreadFunction()
    Dim m_Message As String = ""
    Do
      If MailSlotReadFirstInfinite(m_Message) Then _
       Me.Invoke(New MailSlotDelegate(AddressOf MailSlotUIFunction), m_Message)
    Loop
  End Sub

  Private Sub MailSlotUIFunction(ByVal NewMessage As String)
    '---обработка сообщения---
    'MsgBox(NewMessage)
    Me.Text = NewMessage
    '---обработка сообщения---
  End Sub

  Private Sub Form1_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
    If Not IsNothing(MailSlotThread) Then MailSlotThread.Abort()
    CloseMailSlot() 'необх. нет, т.к. он уничтожится по любому вместе с закрытием приложения
  End Sub
...===========
  Public Function MakeMailSlot(ByVal lpszSlotName As String, _
   Optional ByVal WaitForever As Boolean = False) As Boolean
...
    hMailSlotHandle = CreateMailslot(lpszSlotName, 0, IIf(WaitForever, MAILSLOT_WAIT_FOREVER, 0), m_tSA)

  Public Function MailSlotReadFirstInfinite(ByRef m_Message As String) As Boolean
    'возвращает True,если сообщение прочитано
    'm_Message -возвращает текст первого сообщения при успехе
    'пока не получено, ReadFile ждет за счет MAILSLOT_WAIT_FOREVER при создании MailSlot

    Dim lBufferSize As Integer = 260 ' arbitrary - can be set higher/lower if desired
    Dim sBuffer(lBufferSize - 1) As Byte
    Dim lBytesRead As Integer 'сколько байт прочитали -чтоб обрезать буфер

    ' Read data: optional параметры ф-ции не используем
    If ReadFile(hMailSlotHandle, sBuffer, lBufferSize, lBytesRead, IntPtr.Zero) = False Then
      If Err.LastDllError = ERROR_INSUFFICIENT_BUFFER Then
        GetMailslotInfo(hMailSlotHandle, 0, lBufferSize, 0, 0) 'получаем точный размер буфера, если не хватило
        ReDim sBuffer(lBufferSize - 1)
        'читаем повторно с точным размером буфера
        If ReadFile(hMailSlotHandle, sBuffer, lBufferSize, lBytesRead, IntPtr.Zero) = False Then
          Return False 'хз-не должно такого быть
        End If
      Else
        Return False 'любая другая ошибка, например при обращении к несуществующему MailSlot
      End If
    Else 'в случае успеха с первого раза буфер надо обрезать
      ReDim Preserve sBuffer(lBytesRead - 1)
    End If

    Using mstream As New IO.MemoryStream(sBuffer)
      Using reader As New IO.StreamReader(mstream, System.Text.Encoding.Default, True)
        m_Message = reader.ReadToEnd()
      End Using
    End Using
    Return True
  End Function

  Private Sub Form1_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
   CloseMailSlot() 'необх. нет, т.к. он уничтожится по любому вместе с закрытием приложения
  End Sub


  Private Sub MailSlotThreadFunction()
    Dim m_Message As String = ""
    Do
      If MailSlotReadFirstInfinite(m_Message) Then
        Me.Invoke(New MailSlotDelegate(AddressOf MailSlotUIFunction), m_Message)
      Else
        Exit Do
      End If
    Loop
  End Sub