VB.NET Sending Strings via Postmessage - vb.net

Question

OK, this problem has been bugging me for a long time.
I have the code for the service which communicates a string to the client via PostMessage:
Public Sub SendToClient(msgs As String, types As Integer, hwnd As Long)
postMessage(hwnd, 0, Nothing, msgs)
End Sub
Then I have the client that receives the string:
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
If (m.Msg = 0) Then
Dim a as string
a = Marshal.PtrToStringAuto(m.LParam)
end if
MyBase.WndProc(m)
End Sub
However, the client sends an error, or some jumble of binary data or even just a blank string sometimes. By the way, the m.LParam is a number.
Can someone tell me what is the right way to send/receive strings via postmessage.

Answer 1

I do it like this:
Public Sub SendMessageToApp(ByVal NombreVentana As String, ByVal Mensaje As String, ByVal sender As Form)
Dim hWnd As IntPtr
Dim mCopyData As COPYDATASTRUCT
hWnd = CType(FindWindow(Nothing, NombreVentana), IntPtr)
Dim message As New System.Text.StringBuilder
If (CInt(hWnd) <> 0) Then
message.Append(Mensaje)
Dim pCopyData As IntPtr = Marshal.AllocHGlobal(message.Length() + 40)
mCopyData.lpData = Marshal.StringToHGlobalAnsi(message.ToString)
mCopyData.cbData = message.Length
mCopyData.dwData = CType(_messageID, IntPtr)
Marshal.StructureToPtr(mCopyData, pCopyData, False)
SendMessage(hWnd, WM_COPYDATA, CInt(sender.Handle), pCopyData)
Marshal.FreeHGlobal(mCopyData.lpData)
Marshal.FreeHGlobal(pCopyData)
End If
End Sub
Receiver window:
Declarations and definitions:
Const WM_COPYDATA As Integer = 74
Const SIG_LENGTH As Integer = 36
Const MAX_COPY_LENGTH As Integer = 128
Const SigConnect As String = "F7B82657-BD18-4ee6-B182-78721293821C"
Dim CDCount As Integer
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Integer
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
(ByVal hwnd As IntPtr, ByVal wMsg As Integer, ByVal wParam As Integer, _
ByVal lParam As IntPtr) As Integer
Dim hWndSender As Integer
Private Const _messageID As Integer = 10
'Estructura obligatoria para poder utilizar el API CopyData
<StructLayout(LayoutKind.Sequential)> _
Private Structure COPYDATASTRUCT
Public dwData As IntPtr
Public cbData As Integer
Public lpData As IntPtr
End Structure
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
If m.Msg = WM_COPYDATA Then
Dim Estructura As COPYDATASTRUCT
Dim tipo As Type = Estructura.GetType
Dim message As String
Estructura = CType(m.GetLParam(GetType(COPYDATASTRUCT)), COPYDATASTRUCT)
'Here you get the message
message = Marshal.PtrToStringAnsi(Estructura.lpData, Estructura.cbData)
End If