How to send a structure from VB .NET to C++ with SendMessage?

[Crusader2010]

Hello. I am trying to send a structure from VB .NET 2008 framework 3.5 to an application made in VC++ 6.0, using SendMessage and WM_COPYDATA. Here is the code:

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)> _
    Public Structure MsgSTR
        Public code As String
        Public index1 As Integer
        Public index2 As Integer
        Public arr() As Integer
    End Structure

<StructLayout(LayoutKind.Sequential)> _
    Public Structure CopyData
        Public dwData As IntPtr
        Public cbData As Integer
        Public lpData As MsgSTR
    End Structure

Private Declare Auto Function SendMessage Lib "user32" _
     (ByVal hWnd As IntPtr, _
      ByVal Msg As Integer, _
      ByVal wParam As IntPtr, _
      ByVal lParam As CopyData) As Boolean

    Private Declare Auto Function FindWindow Lib "user32" _
     (ByVal lpClassName As String, _
      ByVal lpWindowName As String) As IntPtr

    Private Const WM_COPYDATA As Integer = &H4A

Private Function mesajSMSSTR(ByVal msg As MsgSTR) As Boolean
        Dim ClientWindow As IntPtr
        Dim a() As System.Diagnostics.Process = System.Diagnostics.Process.GetProcesses

        For Each p In a
            If UCase(p.ProcessName) = "SOMEPROCESS" Then
                ClientWindow = p.MainWindowHandle
                Exit For
            End If
        Next
       
        If Not ClientWindow.Equals(IntPtr.Zero) Then

            Dim message As MsgSTR = msg
            Dim data As CopyData

            data.dwData = ClientWindow
            data.lpData = message
            data.cbData = Marshal.SizeOf(message)

            If frmTest.SendMessage(ClientWindow, frmTest.WM_COPYDATA, Me.Handle, data) then
                Return True
            Else
                Return False
            End If
        Else
            Return False
        End If
End Function

Private Sub cmdSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdSend.Click
       
            Dim msg As MsgSTR
            msg.code = Trim(txtCode.Text)
            msg.index1 = CInt(Trim(txtI1.Text))
            msg.index2 = CInt(Trim(txtI2.Text))
            ReDim msg.arr(40)
            For i = 0 To Trim(txtArr.Text).Split(" ").Length - 1
                msg.arr(i) = CInt(Trim(txtArr.Text).Split(" ")(i))
            Next
         
            If mesajSMSSTR(msg) Then
                MsgBox("Message sent")
            Else
                MsgBox("Message not sent. Process not running.")
            End If
       
End Sub

When i execute the code above, only the string is sent. If i comment the lines related to Arr() (both in the structure and in the _click event), only the first 16 characters of that string are being sent.

For testing purposes, the receiving application uses memcopy to get any data that is sent to it and displays it in a message box.

If i put 3 strings in my structure instead, only the first is sent. I've also tried sending simple integer values, and the message gets sent, but the memory of the vc++ application remains empty and nothing is displayed in the message box.

So basically i cannot send structures nor integers. I've tried marshalling with and without SizeConst and it only seems to solve the issue with only 16 characters being shown.

I've also tried allocating memory for an integer variable, returning an IntPtr and reading the value from this address in the call to my function, but the result is the same - the value is not sent.

Anyone knows what i'm doing wrong or why these two applications cannot communicate properly? Thank you.