Why does CMainFrame::OnMdiNext is not called?

Question

Why does CMainFrame::OnMdiNext is not called?

Flaviu_ 1,071

I have a MFC MDI app. I want to override WM_MDINEXT message. And I did:

header:

protected:
afx_msg int OnCreate(LPCREATESTRUCT lpCreateStruct);
afx_msg LRESULT OnMdiNext(WPARAM wParam, LPARAM lParam);
DECLARE_MESSAGE_MAP()

implementation:

BEGIN_MESSAGE_MAP(CMainFrame, CMDIFrameWndEx)
ON_WM_CREATE()
ON_MESSAGE(WM_MDINEXT, &CMainFrame::OnMdiNext)
END_MESSAGE_MAP()

and

LRESULT CMainFrame::OnMdiNext(WPARAM wParam, LPARAM lParam)
{
TRACE(_T("\n\nWM_MDINEXT intercepted!\n\n"));
MDINext();
// Default behavior:
return __super::DefWindowProc(WM_MDINEXT, wParam, lParam);
}

But when I switch child frames using Ctrl+Tab key, no WM_MDINEXT event fired, even if the child frames are switching ... why? Can you help here?

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Answer 1

RLWA32 50,066

You can get WM_MDINEXT by subclassing the MDI client window. For example --

In CMainFrame class -

public:
	static LRESULT CALLBACK MDIClientProc(HWND, UINT, WPARAM, LPARAM, UINT_PTR, DWORD_PTR);

In CMainFrame::OnCreate -

int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
	if (CMDIFrameWndEx::OnCreate(lpCreateStruct) == -1)
		return -1;

	SetWindowSubclass(m_hWndMDIClient, MDIClientProc, 42, (DWORD_PTR) this);
//....Rest of code
}

Subclass Proc -

LRESULT CMainFrame::MDIClientProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam, UINT_PTR uiSubclass, DWORD_PTR dwRefData)
{
	if (msg == WM_MDINEXT)
		TRACE(_T("Got WM_MDINEXT\n"));
	else if (msg == WM_NCDESTROY)
		RemoveWindowSubclass(hwnd, MDIClientProc, uiSubclass);

	return DefSubclassProc(hwnd, msg, wParam, lParam);
}

Flaviu_ 1,071 Reputation points

2025-07-17T08:43:45.2966667+00:00

I have one small question: what is that magic number? 42

From here: SetWindowSubclass(m_hWndMDIClient, MDIClientProc, 42, (DWORD_PTR) this);
RLWA32 50,066 Reputation points

2025-07-17T10:29:14.4033333+00:00

It has no special meaning other than in literature. :)

If you are a fan of "The Hitchhiker's Guide to the Galaxy" it is the answer given by the supercomputer to "the Ultimate Question of Life, the Universe, and Everything."

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Why does CMainFrame::OnMdiNext is not called?

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