引言

继上篇文章“VC CreateWindow 找不到窗口类”中将RegisterClassEx改为RegisterClass问题就解决了，究竟是什么原因，RegisterClass和RegisterClassEx有什么区别呢？
经过一番查找和测试，终于找到了答案：

不同之处：

The RegisterClass function has been superseded by the RegisterClassEx function. You can still use RegisterClass, however, if you do not need to set the class small icon. In addition,their parameters are different.
E文看不太懂，经过测试就是一个参数的区别，如果使用RegisterClassEx 则WNDCLASSEX的hIconSm参数是必须设置的。

相关代码片：

//函数：MyRegisterClass
//作用：注册窗口类
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wndclass;
    wndclass.cbSize = sizeof(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 = NULL;
    wndclass.lpszClassName = szAppName;
    //将下面这条语句注释，“CreateWindow”时就会出现“找不到窗口类”的错误；
    wndclass.hIconSm = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
    return RegisterClassEx(&wndclass);
}
//函数：InitInstance
//作用：初始化应用程序
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
    HWND hwnd = NULL;
    hwnd = CreateWindow(szAppName, _T("按钮设计"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hInstance, NULL);
    if (hwnd == NULL)
    {
        TCHAR szBuf[128];
        LPVOID lpMsgBuf;
        DWORD dw = GetLastError();
        FormatMessage(
            FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
            NULL,
            dw,
            MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
            (LPTSTR) &lpMsgBuf,
            0, NULL );
        MessageBox(NULL, (LPTSTR)lpMsgBuf, "hwnd", MB_OK);
    }
    BOOL ret = ShowWindow(hwnd, SW_SHOWNORMAL);
    UpdateWindow(hwnd);
    return TRUE;
}