Hi,

I am trying to avoid flicker in this situation:

1) I create a simple VB6 Window application (a void form) and I define
this code for the form (that has the autoredraw property set to false):

'NOTE: this declaration is in another file
Declare Sub InstallMsgHandler Lib _
"E:\TestMsgHandler\Debug\TestMsgHandler.dll" (ByVal lpPC As Long)

Private Sub Form_Load()
    InstallMsgHandler (Me.hWnd)
End Sub

Private Sub Form_Paint()
    Dim lRet As Long
    Dim rectClient As RECT
    lRet = GetClientRect(Me.hWnd, rectClient)

    Dim hTempDC As Long
    hTempDC = GetDC(Me.hWnd)
    Dim hbr, hbrOld As Long
    hbr = CreateSolidBrush(RGB(255, 0, 0)) 'red
    lRet = FillRect(hTempDC, rectClient, hbr)
    lRet = DeleteObject(hbr)
    lRet = ReleaseDC(Me.hWnd, hTempDC)
End Sub

2) I create a MFC dll and I add a CWnd derived class that subclass the
VB form window:

//memdc.h - //header/implementation for double buffer drawing : see
CODEGURU or CODEPROJECT

#ifndef _MEMDC_H_
#define _MEMDC_H_

//////////////////////////////////////////////////
// CMemDC - memory DC
//
// Author: Keith Rule
// Email:  keithr@europa.com
// Copyright 1996-1997, Keith Rule
//
// You may freely use or modify this code provided this
// Copyright is included in all derived versions.
//
// This class implements a memory Device Context

class CMemDC : public CDC {
private:
    CBitmap*    m_bitmap;
    CBitmap*    m_oldBitmap;
    CDC*        m_pDC;
    CRect        m_rcBounds;
public:
    CMemDC(CDC* pDC, const CRect& rcBounds) : CDC()
    {
        CreateCompatibleDC(pDC);
        m_bitmap = new CBitmap;
        m_bitmap->CreateCompatibleBitmap(pDC, rcBounds.Width(),
rcBounds.Height());
        m_oldBitmap = SelectObject(m_bitmap);
        m_pDC = pDC;
        m_rcBounds = rcBounds;
    }
    ~CMemDC()
    {
        m_pDC->BitBlt(m_rcBounds.left, m_rcBounds.top, m_rcBounds.Width(),
m_rcBounds.Height(),
                    this, m_rcBounds.left, m_rcBounds.top, SRCCOPY);
        SelectObject(m_oldBitmap);
        if (m_bitmap != NULL) delete m_bitmap;
    }
    CMemDC* operator->() {
        return this;
    }
};

#endif

// CMyCWnd.h

class CMyCWnd : public CWnd
{
    DECLARE_DYNAMIC(CMyCWnd)

public:
    CMyCWnd();
    virtual ~CMyCWnd();

   bool init(HWND hwnd);

protected:
    DECLARE_MESSAGE_MAP()
public:
   afx_msg void OnPaint();
   afx_msg BOOL OnEraseBkgnd(CDC* pDC);
};

// MyCWnd.cpp : implementation file
//

#include "stdafx.h"
#include "TestMsgHandler.h"
#include "MyCWnd.h"

#include "memdc.h"

extern "C" void WINAPI InstallMsgHandler(HWND hwnd)
{
   static bool bAlreadyInit = false;
   if (!bAlreadyInit)
   {
      static CMyCWnd wnd;
      bAlreadyInit = wnd.init(hwnd);
   }
}

// CMyCWnd

IMPLEMENT_DYNAMIC(CMyCWnd, CWnd)
CMyCWnd::CMyCWnd()
{
}

CMyCWnd::~CMyCWnd()
{
}

bool CMyCWnd::init(HWND hwnd)
{
   return SubclassWindow(hwnd) != 0;
}

BEGIN_MESSAGE_MAP(CMyCWnd, CWnd)
   ON_WM_PAINT()
   ON_WM_ERASEBKGND()
END_MESSAGE_MAP()

void CMyCWnd::OnPaint()
{
   CRect rectClient;
   GetClientRect(&rectClient);

   CPaintDC dc(this); // device context for painting
   CMemDC memdc(&dc, rectClient);

   Default(); // this call the visual basic application redraw.

   rectClient.DeflateRect(rectClient.Width()/4, rectClient.Height()/4);
   CBrush Brush(RGB(0x80,0x80,0x80));
   memdc.FillRect(&rectClient, &Brush);
}

BOOL CMyCWnd::OnEraseBkgnd(CDC* /*pDC*/)
{
   return FALSE;
}

Running this example I have that VC++ draws on the memory buffer while
VB6 draws on the screen. How may I force VB6 to draw on the replaced
in-memory bitmap ?

Thanks for help.
Marco.