Появление MessageBox

#include <Windows.h>
 
#define ID_GROUP_BOX_1 1
#define ID_BUTTON_1    2
#define ID_BUTTON_2    3
 
 
LRESULT CALLBACK WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
int CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
 
    HWND hWnd;
    MSG  uMsg;
 
    WNDCLASSEX mainWin;
    memset(&mainWin, 0, sizeof(WNDCLASSEX));
    mainWin.cbSize = sizeof(mainWin);
    mainWin.hCursor = LoadCursor(NULL, IDC_ARROW);
    mainWin.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    mainWin.lpfnWndProc = WindowProc;
    mainWin.lpszClassName = "FirstClass";
    mainWin.hInstance = hInstance;
    mainWin.style = CS_HREDRAW | CS_VREDRAW;
 
    RegisterClassEx(&mainWin);
 
    if(FindWindow(mainWin.lpszClassName, "My first c++ trainer")!=NULL) 
                    {MessageBox(NULL, "Нельзя запускать больше одного экземпляра приложения",0,0); ExitProcess(0);}
 
    hWnd = CreateWindow(mainWin.lpszClassName, "My first c++ trainer",WS_POPUPWINDOW | WS_CAPTION, 250, 250,400, 300, NULL, NULL, NULL, NULL);
 
    ShowWindow(hWnd, nCmdShow);
 
    while(GetMessage(&uMsg, NULL, 0 ,0))
    {
        TranslateMessage(&uMsg);
        DispatchMessage(&uMsg);
    }
 
 
    return uMsg.wParam;
}
 
 
LRESULT CALLBACK WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
 
    switch (uMsg)
    {
    case WM_CLOSE:
        ExitProcess(0);
        break;
 
    case WM_CREATE: {
        HWND groupBox_hWnd = 
        CreateWindow("BUTTON", "Группа кнопок", WS_CHILD | WS_VISIBLE | BS_GROUPBOX, 10 ,10, 150, 100, hWnd, (HMENU)ID_GROUP_BOX_1, NULL, NULL);
        CreateWindow("BUTTON", "Верхняя кнопка", WS_CHILD | WS_VISIBLE, 10, 30, 130, 20, groupBox_hWnd, (HMENU)ID_BUTTON_1, NULL, NULL);
        CreateWindow("BUTTON", "Нижняя кнопка", WS_CHILD | WS_VISIBLE, 10, 60, 130, 20, groupBox_hWnd, (HMENU)ID_BUTTON_2, NULL, NULL);
        break;
        }
 
    case WM_COMMAND:
        switch(wParam)
        {
        case ID_BUTTON_1:
            MessageBox(hWnd, "Pushed button 1", "Push", MB_OK);
            break;
 
        case ID_BUTTON_1:
            MessageBox(hWnd, "Pushed button 2", "Push", MB_OK);
            break;
        }
        break;
 
    default:  return DefWindowProc(hWnd, uMsg, wParam, lParam);
        break;
    }
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

            DefGroupBoxProc = (WNDPROC)GetWindowLongPtr(groupBox_hWnd, GWLP_WNDPROC);
            SetWindowLongPtr(groupBox_hWnd, GWLP_WNDPROC,(LPARAM)NewGroupBoxProc);