一个WINDOWS程序设计书上的简单例子，自己修改了一下，有一点不太清楚！

[yiruirui0507]

#include <windows.h>

#define ID_TIMER    1

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PSTR szCmdLine, int iCmdShow)
{
     static TCHAR szAppName[] = TEXT ("Beeper1") ;
     HWND         hwnd ;
     MSG          msg ;
     WNDCLASS     wndclass ;
     
     wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
     wndclass.lpfnWndProc   = WndProc ;
     wndclass.cbClsExtra    = sizeof(long) ;
     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 ;
     
     if (!RegisterClass (&wndclass))
     {
          MessageBox (NULL, TEXT ("Program requires Windows NT!"), 
                      szAppName, MB_ICONERROR) ;
          return 0 ;
     }
     
     hwnd = CreateWindow (szAppName, TEXT ("Beeper1 Timer Demo"),
                          WS_OVERLAPPEDWINDOW,
                          CW_USEDEFAULT, CW_USEDEFAULT,
                          CW_USEDEFAULT, CW_USEDEFAULT,
                          NULL, NULL, hInstance, NULL) ;
          
     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)
{
     static BOOL fFlipFlop = FALSE ;
     HBRUSH      hBrush ;
     HDC         hdc ;
     PAINTSTRUCT ps ;
     RECT        rc ;
     
     switch (message)
     {
     case WM_CREATE:
          SetTimer (hwnd, ID_TIMER, 1000, NULL) ;
          return 0 ;

     case WM_TIMER :
          MessageBeep (-1) ;          
          fFlipFlop = !fFlipFlop ;
		  //SetClassLong(hwnd,GCL_HCURSOR,fFlipFlop ? (long)LoadCursor(NULL,IDC_CROSS):LoadCursor(NULL,IDC_ARROW));
		  //UpdateWindow(hwnd);
		  SetCursor(fFlipFlop ? LoadCursor(NULL,IDC_CROSS):LoadCursor(NULL,IDC_ARROW));
          InvalidateRect (hwnd, NULL, FALSE) ;
          return 0 ;
          
     case WM_PAINT :
          hdc = BeginPaint (hwnd, &ps) ;
          
          GetClientRect (hwnd, &rc) ;
          hBrush = CreateSolidBrush (fFlipFlop ? RGB(255,0,0) : RGB(0,0,255)) ;
          FillRect (hdc, &rc, hBrush) ;
		  //SetCursor(fFlipFlop ? LoadCursor(NULL,IDC_CROSS):LoadCursor(NULL,IDC_ARROW));
          
		  EndPaint (hwnd, &ps) ;
          DeleteObject (hBrush) ;
          return 0 ;
          
     case WM_DESTROY :
          KillTimer (hwnd, ID_TIMER) ;
          PostQuitMessage (0) ;
          return 0 ;
     }
     return DefWindowProc (hwnd, message, wParam, lParam) ;
}

以上代码是正确的，设置了一个定时器，每隔一秒变换一下背景画刷跟鼠标指针，用的是SetCursor(fFlipFlop ? LoadCursor(NULL,IDC_CROSS):LoadCursor(NULL,IDC_ARROW));
这个函数可以实现，没问题，
我现在想用SetClassLong是完成这个功能，MSDN说需要specified offset into the extra class memory
所以我设置了 wndclass.cbClsExtra = sizeof(long) ;
然后调用 SetClassLong(hwnd,GCL_HCURSOR,fFlipFlop ? (long)LoadCursor(NULL,IDC_CROSS):LoadCursor(NULL,IDC_ARROW));
本来以为没什么问题了，可运行效果没体现出来，只有在背景画刷上移动才能看到效果，请问我哪里写错了？

[wltg2001]

用SetClassLong并不会马上生效，要等下一次重绘时才会出效果。你可以用UpdateWindow强制重绘

[wltg2001]

其实MSDN上有说明的：
Use the SetClassLong function with care. For example, it is possible to change the background color for a class by using SetClassLong, but this change does not immediately repaint all windows belonging to the class.

[yiruirui0507]

[Quote=引用 2 楼 wltg2001 的回复:]
其实MSDN上有说明的：
Use the SetClassLong function with care. For example, it is possible to change the background color for a class by using SetClassLong, but this change does not immediately repaint all wi……
[/Quote]

首先感谢你的回答，加上UPDATEWINDOW也不行的，我加了的，你看上面代码注释的部分就晓得了。

[wltg2001]

[Quote=引用 3 楼 yiruirui0507 的回复:]
引用 2 楼 wltg2001 的回复:
其实MSDN上有说明的：
Use the SetClassLong function with care. For example, it is possible to change the background color for a class by using SetClassLong, but this change does not imme……
[/Quote]
试了一下，SetClassLong似乎就是这样的，要移一下鼠标才会发现效果。
另外，wndclass.cbClsExtra = sizeof(long);这个倒是不用加的。

[yiruirui0507]

[Quote=引用 4 楼 wltg2001 的回复:]
引用 3 楼 yiruirui0507 的回复:
引用 2 楼 wltg2001 的回复:
其实MSDN上有说明的：
Use the SetClassLong function with care. For example, it is possible to change the background color for a class by using SetClassLong, but……
[/Quote]
恩，这个的确不需要加，难道真的SETCLASSLONG就这么无能吗？MY GOD！！！