How can I center a Dialog Box over main program window position? - c

Question

I've got this code to open an InputBox defined on a DLL that get HMODULE that I save on hInstance variable when main program calls. How can I center it over the main program window?
It occurs that doesn't work and shows the DialogBox on top left on Screen or on top left of program window randomly.
#include <windows.h>
#include "resource.h"
char IB_res[10];
double defaultValue = 0;
BOOL CALLBACK InputBox_WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_INITDIALOG:
if (defaultValue != -1)
SetDlgItemText(hwnd, IDC_EDIT, (LPCSTR)(my_printf("%f", defaultValue).c_str()));
else
SetDlgItemText(hwnd, IDC_EDIT, (LPCSTR)"");
return TRUE;
case WM_COMMAND:
switch(LOWORD(wParam))
{
case IDOK:
if (!GetDlgItemText(hwnd, IDC_EDIT, IB_res, 10))
*IB_res = 0;
case IDCANCEL:
EndDialog(hwnd, wParam);
break;
}
break;
default:
return FALSE;
}
return TRUE;
}
DWORD processId;
HWND hwndParent;
BOOL CALLBACK enumWindowsProc(HWND hwnd, LPARAM lParam)
{
DWORD procid;
GetWindowThreadProcessId(hwnd, &procid);
if (procid == processId)
hwndParent = hwnd;
return TRUE;
}
HINSTANCE hInstance;
const char* InputBox(double def_value)
{
defaultBetValue = def_value;
processId = GetCurrentProcessId();
EnumWindows(enumWindowsProc, 0);
INT_PTR ret = DialogBox(hInstance, MAKEINTRESOURCE(IDD_IB), hwndParent, InputBox_WndProc);
DWORD error = GetLastError();
if (ret != IDOK)
*IB_res = 0;
return IB_res;
}

Answer 1

From:
http://msdn.microsoft.com/en-gb/library/windows/desktop/ms644996(v=vs.85).aspx
case WM_INITDIALOG:
// Get the owner window and dialog box rectangles.
if ((hwndOwner = GetParent(hwndDlg)) == NULL)
{
hwndOwner = GetDesktopWindow();
}
GetWindowRect(hwndOwner, &rcOwner);
GetWindowRect(hwndDlg, &rcDlg);
CopyRect(&rc, &rcOwner);
// Offset the owner and dialog box rectangles so that right and bottom
// values represent the width and height, and then offset the owner again
// to discard space taken up by the dialog box.
OffsetRect(&rcDlg, -rcDlg.left, -rcDlg.top);
OffsetRect(&rc, -rc.left, -rc.top);
OffsetRect(&rc, -rcDlg.right, -rcDlg.bottom);
// The new position is the sum of half the remaining space and the owner's
// original position.
SetWindowPos(hwndDlg,
HWND_TOP,
rcOwner.left + (rc.right / 2),
rcOwner.top + (rc.bottom / 2),
0, 0, // Ignores size arguments.
SWP_NOSIZE);
if (GetDlgCtrlID((HWND) wParam) != ID_ITEMNAME)
{
SetFocus(GetDlgItem(hwndDlg, ID_ITEMNAME));
return FALSE;
}
return TRUE;