Private Sub form1_Load(ByVal sender As System.Object, ByVal e As _
     System.EventArgs) Handles MyBase.Load
        Dim hwnd As Integer = getWindowHandle()
        If hwnd = -1 Then Return
        Try
            MsgBox("Sending Msg")
            SendMessage(hwnd, WM_APP, 0, 0)
        Finally
        End Try
    End Sub
    Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" ( _
      ByVal hwnd As Int32, ByVal wMsg As Int32, ByVal wParam As Int32, ByVal _
      lParam As Integer) As Int32
    Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" (ByVal _
    lpClassName As String, ByVal lpWindowName As String) As Int32
    Const WM_COPYDATA As Integer = &H4A
    Private Const WM_APP As Int32 = &H8000
 
    Private Function getWindowHandle() As Integer
        Dim p As Process
        For Each p In System.Diagnostics.Process.GetProcessesByName( _
          Process.GetCurrentProcess.ProcessName)
           If p.Id <> Process.GetCurrentProcess.Id AndAlso _
           p.MainWindowTitle = Me.Text Then
                Return p.MainWindowHandle.ToInt32
            End If
        Next
        Return -1
    End Function
 
    Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        MyBase.WndProc(m)
        Try
            If m.Msg = WM_APP Then
                MessageBox.Show("Nachricht: ")
            End If
        Finally
        End Try
    End Sub