How to validate CEdit only number input?

I use Visual C++ 4.2 Professional.

I want to validate so that user only able to input numeric values. I've
looked for the keydown notify message, but I could not find one.

I've tried to trap the input by deriving a new class named CEditExt from
CEdit and make OnChar event but it did not work, I've debugged it but when I
press any non-numeric key, the program sequence did not entering the OnChar
event.

What is wrong?

I've attached my codes below:

Thank you very much.

in .cpp file:

// CEditExt
IMPLEMENT_DYNAMIC(CEditExt, CEdit)

BEGIN_MESSAGE_MAP(CEditExt, CEdit)
    ON_WM_CHAR()
END_MESSAGE_MAP()
// CEditExt ??????????????? ????????????

void CEditExt::OnChar( UINT nChar, UINT nRepCnt, UINT nFlags )
{
    if( nChar < '0' || nChar > '9' )
    {
        if ( nChar != VK_BACK )
            return;
    }
    CEditExt::OnChar( nChar, nRepCnt, nFlags );
}

in .h file:
class CEditExt : public CEdit
{
    DECLARE_DYNAMIC( CEditExt )

public:
    CEditExt();
    virtual ~CEditExt();

protected:
    DECLARE_MESSAGE_MAP()

public:
    afx_msg void OnChar( UINT nChar, UINT nRepCnt, UINT nFlags );
};