MessageBox return without user input? - winapi

Question

I am trying to implemente a close-save-file dialog into a Win32 project, but encouter a strange problem. Here is my solution.
Create a simple win32 project in Visual Studio.
Handle WM_COMMAND to create new window.
case WM_COMMAND:
wmId = LOWORD(wParam);
wmEvent = HIWORD(wParam);
// Parse the menu selections:
switch (wmId)
{
case IDM_ABOUT:
InitInstance(hInst, SW_SHOW);
//DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
break;
case IDM_EXIT:
windowCount--;
DestroyWindow(hWnd);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
break;
Handle WM_SYSCOMMNAD to show the save-before-quit messagebox.
case WM_SYSCOMMAND:
if (wParam == SC_CLOSE) {
int ret = MessageBox(NULL, L"do you really want to close", L"question", MB_YESNO|MB_APPLMODAL);
if (ret == IDNO)
return 0;
closedCount++;
StringCchPrintf(buff, 256, L"hwnd %x user choose to close\n", hWnd);
OutputDebugString(buff);
}
return DefWindowProc(hWnd, message, wParam, lParam);
two variable closeCount and windowCount to ensure terminate after all windows are closed.
// Main message loop:
while (GetMessage(&msg, NULL, 0, 0) || (closedCount != windowCount))
{
if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
HWND hWnd;
hInst = hInstance; // Store instance handle in our global variable
hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
if (!hWnd)
{
return FALSE;
}
windowCount++;
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
return TRUE;
}
Full code: http://pastebin.com/EVWWMz8L
There are two bugs in above code:
Create two windows and click close button for each window, then confirm to close for one window would close both windows. Which means, close MessageBox in one window would cause MessageBox in the other window return without any user input.
Create two windows and click close button for each window. Then use aero thumbnail to activate one messagebox and confirm to close, but the associated window would not close. I need to confirm both messagebox to close window.
How could this happen, what's wrong with my code?

Answer 1

1, Handler for WM_DESTROY is wrong, right handler should be:
case WM_DESTROY: {
StringCchPrintf(buff, 256, L"%x recieved Wm-DESTROY\n", hWnd);
OutputDebugString(buff);
closedCount++;
if (closedCount == windowCount) {
PostQuitMessage(0);
}
}
FULL code: http://pastebin.com/EU7cVKUb
Root cause:
PostQuitMessage terminate the MessageBox/DialogBox message loop, cause them missing and return immediate. In conclusion, DO NOT call PostQuitMessage until you really need to do this.
2, MessageBox/DialogBox should have owner window, otherwise user could use alt+tab/aero-thumbnail to select the box. So create the messagebox with hwnd as parent could solve problem 2.