Re: CWnd-derived class, VC6SP6, Invalid m_hWnd

Jason Tost <>
Wed, 31 May 2006 14:53:28 -0600
Thank you very much for your feedback and suggestions. The information
you gave is appreciated very much. I will continue stepping through the
MFC source and checking values before and after the call to Create().

Again, I appreciate your assistance.

Best regards,

Joseph M. Newcomer wrote:

Beware of using WM_USER messages; they are often preempted by Microsoft. Use WM_APP
messages, or better still, Registered Window Messages. See my essay on Message Management
on my MVP Tips site.

See below....

On Wed, 31 May 2006 11:21:02 -0600, Jason Tost <> wrote:

I'm trying to create a class that will derive from CWnd so that it can
have a message map, however it does not need to display anything. This
class will be a member of a CWnd-derived object (CChildView) (not using
Document/View), and will simply be a target for posting messages to from
various code in the application. The class declaration is as follows:

#define WM_ALERT (WM_USER+1)
class MyWnd : public CWnd {
    afx_msg LRESULT OnAlert(WPARAM w, LPARAM l);

The implementation for this class is as follows:


        _T("Message was successfully posted and dispatched."),
        _T("Test Event"),
    return 0;

The view declaration is as follows:

If this is really a view, in the doc/view sense, it must be derived from CView, not CWnd.

class CChildView : public CWnd
// Construction

// Attributes

// Operations

// Overrides
    // ClassWizard generated virtual function overrides
    virtual BOOL PreCreateWindow(CREATESTRUCT& cs);

// Implementation
    virtual ~CChildView();

    // Generated message map functions

    MyWnd m_MyObj;

    virtual void OnInitialUpdate(); // called first time after construct

    afx_msg void OnPaint();

I am using the OnInitialUpdate() member to create the window as follow:

void CChildView::OnInitialUpdate() {
    // IDC_OBJ_MINE is a resource ID in resource.h
    // and is unique.
    BOOL bResult = m_MyObj.Create(NULL, NULL, WS_CHILD,
        CRect(0, 0, 100, 100),
        this, IDC_OBJ_MINE);
    if (!bResult) {
            _T("Unable to create child window instance."),
            _T("Stupid Thing(tm) anyway..."),
            MB_OK | MB_ICONSTOP);
    if (m_MyObj.GetSafeHwnd() == NULL) {
        CString ErrMsg;
            _T("No HWND is present in child (Error %d)"),

            _T("Stupid Thing(tm) anyway..."),
            MB_OK | MB_ICONSTOP);

When the application runs, the call to CWnd::Create() succeeds, however
the message appears indicating that the m_hWnd/GetSafeHwnd() value is
NULL, and GetLastError() returns 0. Of course, since I get no HWND I am
not able to post messages to the object. Is it possible to create
so-called "invisible" windows like this, and am I missing something
fundamental in the creation of a CWnd-based child class?

I've done this many times in the way you show, or at least I'm not seeing any significant
difference between what you are doing and what I do. You aren't setting WS_VISIBLE, you
are setting WS_CHILD, you are giving 'this' as the parent, and you have a control ID
(although IDC_STATIC might suffice; I don't know where you are using the IDC_OBJ_MINE
other than here).

The fact that the GetSafeHwnd() returns NULL is inconsistent with bResult being TRUE, so I
think there is a fairly serious problem here, but I have no idea short of doing a lot of
single-stepping through MFC exactly what went wrong. What I'd first do is set a
breakpoint immediately after the Create and examine the m_hWnd using the debugger. If it
is NULL at that point, I'd set a breakpoint at the AfxHookProc (or some similar function,
it has been a while since I've looked inside MFC at this level) to make sure the hook
function is being called. Set a breakpoint at the Create call; when that breakpoint is
taken, set a breakpoint in the hook function. The hook function is nominally what does
the binding of the HWND to the CWnd-derived object. If the hook isn't called, I'd start
doing some serious single-stepping through Create looking for something strange. Alas, I
can't point you any better than that.

Thank you all for your efforts and suggestions!


Joseph M. Newcomer [MVP]
MVP Tips:

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