MarshalAs COPYDATASTRUCT and SendMessage

[dvanmil]

I am having problems with Window Messages. I want to send the handle of a VB.NET winform to a VB6 application. The code I have used seems to work ok, since the message arrives at the VB6 app. However, I have a problem that sometimes I get an error stating that it is not possilbe to read or write to the memory and that memory might be corrupt.
I have no idea what is causing this problem as I think I release the memory at the end of my procedure.

I have provided the code in hope that someone else might have some insights in this problem.

Imports System.Runtime.InteropServices
Imports System.Security
    Public Class WindowMessages

#Region "Constants"
        Const WM_COPYDATA = &H4A
        Const LMEM_FIXED As Integer = &H0
        Const LMEM_ZEROINIT As Integer = &H40
        Const LPTR As Integer = (LMEM_FIXED Or LMEM_ZEROINIT)
#End Region

#Region "Dll import"
        <DllImport("user32.dll", EntryPoint:="SendMessageA", CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Ansi)> _
        Private Shared Function SendMessage(ByVal hwnd As UInteger, ByVal Msg_Const As UInteger, ByVal wParam As UInteger, ByVal lParam As UInteger) As UInteger
        End Function

#End Region

        <MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=255)>
        Private Shared text As String

        <StructLayout(LayoutKind.Sequential)>
        Public Structure COPYDATASTRUCT
            Public dwData As IntPtr   ' Use this to identify your message
            Public cbData As Integer   ' Number of bytes to be transferred
            Public lpData As IntPtr   ' Address of data
        End Structure

        Public Shared Sub SendDataMsg(ByVal statusText As String, ByVal lngToHwnd As IntPtr, ByVal lngFromHwnd As IntPtr)
            text = String.Copy(statusText)

            Dim oCDS As New COPYDATASTRUCT()

            oCDS.cbData = (text.Length + 1) 
            oCDS.lpData = Marshal.StringToHGlobalUni(text)
            oCDS.dwData = 3
            Dim lParam As IntPtr = Marshal.AllocHGlobal(oCDS.cbData)
            Marshal.StructureToPtr(oCDS, lParam, False)

            Call SendMessage(lngToHwnd, WM_COPYDATA, lngFromHwnd, lParam)

            Marshal.FreeHGlobal(oCDS.lpData)
            Marshal.FreeHGlobal(lParam)

        End Sub

    End Class