windows程序设计制定系统菜单，为什么运行后的窗口不显示。解决思路

windows程序设计制定系统菜单，为什么运行后的窗口不显示。
#include <windows.h>
#define IDM_SYS_ABOUT 1
#define IDM_SYS_HELP 2
#define IDM_SYS_REMOVE 3
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM) ;
static TCHAR szAppName[] = TEXT("PoorMenu") ;
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
{
     HMENU hMenu;
     HWND hwnd;
     MSG msg;
     WNDCLASS wndclass;
     wndclass.style= CS_HREDRAW| CS_VREDRAW;
     wndclass.lpfnWndProc= WndProc;
     wndclass.cbClsExtra= 0 ;
     wndclass.cbWndExtra= 0 ;
     wndclass.hInstance= hInstance;
     wndclass.hIcon= LoadIcon(NULL, IDI_APPLICATION) ;
     wndclass.hCursor= LoadCursor(NULL, IDC_ARROW) ;
     wndclass.hbrBackground= (HBRUSH) GetStockObject(WHITE_BRUSH) ;
     wndclass.lpszMenuName= szAppName;
     wndclass.lpszClassName= szAppName;
     if (!RegisterClass(&wndclass)) {
           MessageBox( NULL, TEXT("This program requires Windows NT!"),
               szAppName, MB_ICONERROR) ;
          return 0 ;
      }
      hwnd= CreateWindow( szAppName, TEXT("The Poor-Person's Menu"),
                         WS_OVERLAPPEDWINDOW,
                         CW_USEDEFAULT, CW_USEDEFAULT,
                         CW_USEDEFAULT, CW_USEDEFAULT,
                         NULL, NULL, hInstance, NULL) ;
      hMenu= GetSystemMenu(hwnd, FALSE) ;   //得到一个系统菜单的句柄
      AppendMenu(hMenu, MF_SEPARATOR, 0, NULL) ;  //添加分隔条
      AppendMenu(hMenu, MF_STRING, IDM_SYS_ABOUT, TEXT("About...")) ;
      AppendMenu(hMenu, MF_STRING, IDM_SYS_HELP, TEXT("Help...")) ;
      AppendMenu(hMenu, MF_STRING, IDM_SYS_REMOVE, TEXT("Remove Additions")) ;
      ShowWindow(hwnd, iCmdShow) ;
      UpdateWindow(hwnd) ;
      while (GetMessage(&msg, NULL, 0, 0)) {
            TranslateMessage(&msg) ;
            DispatchMessage(&msg) ;
       }
      return msg.wParam;
}
LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wParam,LPARAM lParam)
{
        switch (message) {
        case WM_SYSCOMMAND:
              switch (LOWORD(wParam)) {
                    case IDM_SYS_ABOUT:
                       MessageBox( hwnd, TEXT("A Poor-Person's Menu Program\n")
                                 TEXT("(c) Charles Petzold, 1998"),
                                 szAppName, MB_OK| MB_ICONINFORMATION) ;
                     return 0 ;
                    case IDM_SYS_HELP:
                       MessageBox( hwnd, TEXT("Help not yet implemented!"),
                                  szAppName, MB_OK| MB_ICONEXCLAMATION) ;