SendMessage - Passing a string from VB6 to VB.NET - vb.net

Question

Good afternoon,
I am attempting to use SendMessage to pass a string from a VB6 EXE, to a .NET 2013 EXE. I know that the message is getting in to the .NET EXE, because I'm able to set a breakpoint on it and it comes up when I call SendMessage from the VB6 EXE. The problem I am having is retrieving the string.
This is how I am attempting to do it:
VB6 Code:
Option Explicit
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (dest As Any, Source As Any, ByVal bytes As Long)
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" (ByVal hwnd As Long, ByVal msg As Long, wParam As Long, lParam As Any) As Long
Private Const APPVIEWER_OPEN = &H400
Private Sub Command1_Click()
Dim hwndAppViewer As Long
Dim bytBuffer(1 To 255) As Byte
Dim sParams As String
Dim lStringAddress As Long
hwndAppViewer = FindWindow(vbNullString, "DotNetReceiver")
If hwndAppViewer > 0 Then
sParams = "STRINGDATA"
CopyMemory bytBuffer(1), sParams, Len(sParams)
lStringAddress = VarPtr(bytBuffer(1))
SendMessage hwndAppViewer, APPVIEWER_OPEN, Me.hwnd, lStringAddress
End If
End Sub
Here is the .NET code:
Imports System.Runtime.InteropServices
Public Class Form1
Protected Overrides Sub WndProc(ByRef m As Message)
Dim sPolicyInformation As String
If m.Msg = &H400 Then
sPolicyInformation = Marshal.PtrToStringAnsi(m.LParam)
Else
MyBase.WndProc(m)
End If
End Sub
End Class
The problem comes when I try and retrieve the string. I am getting a blank string. I noticed that the number in the VB6 lStringAddress and the number in .NET m.lParam are completely different, so I must be missing something about how I'm passing the address through lParam.
Any ideas what I might be missing?
Thank you.

Answer 1

You are sending an ANSI string to VB.NET. VB6 was designed for all MS's OSs and 9x wasn't unicode. So all strings passed to API calls will be converted to ANSI. Windows will convert that ANSI string to unicode for the VB.NET program when it recieves it.
Use the sendmessagew function and send the first element of a byte array that's null terminated.
Dim MyStr() as byte
MyStr = "cat" & chrw(0)
The pass only the first element to SendMessageW ie MyStr(0). Windows API uses null terminated C strings. COM and VB6 use BStr (a size header and a non null terminated string).
When passing strings by ref you pass the address of the header. When passing by value you pass the address of the first character (making it a c string if you tack a null on the end).