Callback from C dll to VB6

[moeur]

Has anyone seen a working example of making an asynchronous callback from a C++ dll to VB6?
The steps are:
1. Set callback address by sending addressof subroutine to dll
2. dll stores this value in a global variable for later use
3. dll launches a seperate thread and returns control to VB
4. New thread performs some work and makes a callback to VB using the previously set address.

The problem is with the parameters passed back to VB
The logic works within the IDE
If compiled, the callback is made, but if I try to access the parameters passed, it crashes. This tells me there is something wrong with the stack?

Here is a demo code
C++

/ All declarations which are needed resides in "windows.h"
#include <windows.h>

// Placeholder for function-export macros
#define LIBRARY_EXPORT __declspec(dllexport) __stdcall

// Function Declarations for Static Export
extern "C" {
long LIBRARY_EXPORT GetDBData(void);
long LIBRARY_EXPORT SetCallbackProcAddr(long);
}

// Function Declaration for Callback function of the Calling App
//typedef void (CALLBACK* TDataReadNotify)(long);
typedef void (__stdcall *TDataReadNotify)(long);

/ Vars for Imported Functions
TDataReadNotify DataReadNotify;
HANDLE hThread;
DWORD pThreadId;

static DWORD DoTheWork(LPVOID lpParameter)
{
  static long lDBData;

// Doing stuff that takes 5 sec., i.e. reading DB Data.

  Sleep(5000);
  lDBData = 123;

// Notify the caller app via CB-function.
  if (DataReadNotify != NULL) DataReadNotify(lDBData);

  return 0;
};

long LIBRARY_EXPORT GetDBData(void)
{
  long lProcAdr;

// Get ProcAddress for Thread-Func and create/run the thread.
  lProcAdr = (long)(&DoTheWork);
  hThread = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)lProcAdr, NULL, 0, &pThreadId);

// Return to caller app.
  return 0;
};

long LIBRARY_EXPORT SetCallbackProcAddr(long lProcAddress)
{
  long Result = 0;

// Set the Proc-Address for the notify-func of the caller app.
  DataReadNotify = (TDataReadNotify)lProcAddress; 

  if (DataReadNotify == NULL) return 0;

  return 1;
};

Option Explicit
 
Private iWait As Long
Private sDBData As String
Private sMessage As String
 
Public Declare Function GetDBData Lib "DLL_Callback.dll" Alias "_GetDBData@0" () As Long
Public Declare Function SetCallbackProcAddr Lib "DLL_Callback.dll" Alias "_SetCallbackProcAddr@4" (ByVal lProcAddress As Long) As Long
 
Sub Main()
 
 
  iWait = 1
 
' Send Address-Pointer Information to the DLL
  Call SetCallbackProcAddr(AddressOf DataReadNotify)
' Call the function of the DLL, which reads DB-Info or something else.
  GetDBData
 
' if the DLL-function is still busy, loop until the DLL notifies via the CALLBACK-function "DataReadNotify()".
  Do While iWait = 1
    DoEvents
  Loop
  
  iWait = iWait
 
End Sub
 
Public Sub DataReadNotify(ByVal lDBData As Long)
  MsgBox "Sample App: " & CStr(lDBData), vbOKOnly + vbInformation
 
' Set Wait to 0, so that WinMain can go further.
  iWait = 0
 
End Sub