Why it cant create the child window?

Question

Can anyone tell me why the child window cannot be created? I'm using the forgers win32api guide but I cannot figure out what is the problem.

When the program starts running I have all of the controls, but when I click on the 'new' menuitem I get the error message. This is right after the winmain.

Other things like menuitems, the tool and status bars, opening or saving files works.

**HWND CreateNewMDIChild(HWND hMDIClient)
{
    MDICREATESTRUCT mcs;
    HWND hChild;
    mcs.szTitle = "[Untitled]";
    mcs.szClass = g_szChildClassName;
    mcs.hOwner  = GetModuleHandle(NULL);
    mcs.x = mcs.cx = CW_USEDEFAULT;
    mcs.y = mcs.cy = CW_USEDEFAULT;
    mcs.style = MDIS_ALLCHILDSTYLES;
    hChild = (HWND)SendMessage(hMDIClient, WM_MDICREATE, 0, (LONG)&mcs);

    if(!hChild)
    {
        MessageBox(hMDIClient, "MDI Child creation failed.", "Oh Oh...",
            MB_ICONEXCLAMATION | MB_OK);
    }
    return hChild;

}**

Please post an [MCVE](http://stackoverflow.com/help/mcve) or remove the code of less/no interest. — Mohit Jain, Apr 06 '15 at 09:01
how about look - are your windows procedure(of g_szChildClassName) called at all? how about GetLastError() ? — sutol, Apr 06 '15 at 12:09

score 2 · Accepted Answer · answered Apr 06 '15 at 21:29

That's an unfortunate bug in the sample code, that prevents it from running on 64-bit Windows. The final parameter to SendMessage is of type LPARAM (an alias for LONG_PTR). Casting it to LONG truncates it to 4 bytes, not quite sufficient for a 64-bit pointer (see Data Type Ranges).

Change the following line

hChild = (HWND)SendMessage(hMDIClient, WM_MDICREATE, 0, (LONG)&mcs);

to

hChild = (HWND)SendMessage(hMDIClient, WM_MDICREATE, 0, (LPARAM)&mcs);

and the code should run as expected.

I've made the change but it does not working. I've copypasted the code from the sample file and it works but still don't know how...thanks anyway. — , Apr 07 '15 at 09:09

Why it cant create the child window?

1 Answers1