Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" ( _
        ByVal hwnd As Int32, _
        ByVal wMsg As Int32, _
        ByVal wParam As Int32, _
        ByRef lParam As COPYDATASTRUCT) _
        As Int32
    Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" ( _
    ByVal lpClassName As String, _
    ByVal lpWindowName As String) As Int32
    Public Structure COPYDATASTRUCT
        Public dwData As IntPtr
        Public cbData As Integer
        Public lpData As String
    End Structure
 
    Const WM_COPYDATA As Integer = &H4A
 
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        Dim cpy As COPYDATASTRUCT
        Dim strMsg As String = "Hello World!"
        Dim hwnd As Integer = getWindowHandle()
        cpy.lpData = strMsg
        cpy.cbData = strMsg.Length
        If hwnd = -1 Then MessageBox.Show("Keine 2. Instanz gefunden!") : Return
        Try
            SendMessage(hwnd, WM_COPYDATA, 0, cpy)
        Catch ex As Exception
            MessageBox.Show(ex.ToString)
        End Try
    End Sub
 
 
    Private Function getWindowHandle() As Integer
        For Each p As Process In System.Diagnostics.Process.GetProcessesByName( _
          Process.GetCurrentProcess.ProcessName)
            If p.Id <> Process.GetCurrentProcess.Id AndAlso _
            p.MainWindowTitle = "Destination" 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_COPYDATA Then
                Dim cpy As COPYDATASTRUCT
                cpy = CType(m.GetLParam(cpy.GetType), COPYDATASTRUCT)
                MessageBox.Show("Nachricht: " + cpy.lpData)
            End If
        Catch ex As Exception
            MessageBox.Show(ex.ToString)
        End Try
    End Sub
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        Me.Text = "Destination"
    End Sub