《Windows程序设计》第七章BlockOut2的有关问题

《Windows程序设计》第七章BlockOut2的问题
/*---------------------------------------------------
  BLOKOUT2.C -- Mouse Button & Capture Demo Program
  (c) Charles Petzold, 1998
  ---------------------------------------------------*/

#include <windows.h>

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

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
  PSTR szCmdLine, int iCmdShow)
{
  static TCHAR szAppName[] = TEXT ("BlokOut2") ;
  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 = NULL ;
  wndclass.lpszClassName = szAppName ;
   
  if (!RegisterClass (&wndclass))
  {
  MessageBox (NULL, TEXT ("Program requires Windows NT!"), 
  szAppName, MB_ICONERROR) ;
  return 0 ;
  }
   
  hwnd = CreateWindow (szAppName, TEXT ("Mouse Button & Capture 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 ;
}

void DrawBoxOutline (HWND hwnd, POINT ptBeg, POINT ptEnd)
{
  HDC hdc ;
   
  hdc = GetDC (hwnd) ;
   
  SetROP2 (hdc, R2_NOT) ;
  SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
  Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
   
  ReleaseDC (hwnd, hdc) ;
}

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
  static BOOL fBlocking, fValidBox ;
  static POINT ptBeg, ptEnd, ptBoxBeg, ptBoxEnd ;
  HDC hdc ;
  PAINTSTRUCT ps ;
   
  switch (message)
  {
  case WM_LBUTTONDOWN :
  ptBeg.x = ptEnd.x = LOWORD (lParam) ;
  ptBeg.y = ptEnd.y = HIWORD (lParam) ;
   
  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
   
  SetCapture (hwnd) ;
  SetCursor (LoadCursor (NULL, IDC_CROSS)) ;
   
  fBlocking = TRUE ;
  return 0 ;
   
  case WM_MOUSEMOVE :
  if (fBlocking)
  {
  SetCursor (LoadCursor (NULL, IDC_CROSS)) ;
   
  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
   
  ptEnd.x = LOWORD (lParam) ;
  ptEnd.y = HIWORD (lParam) ;
   
  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

  }
  return 0 ;
   
  case WM_LBUTTONUP :
  if (fBlocking)
  {
  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
   
  ptBoxBeg = ptBeg ;
  ptBoxEnd.x = LOWORD (lParam) ;
  ptBoxEnd.y = HIWORD (lParam) ;
   
  ReleaseCapture () ;
  SetCursor (LoadCursor (NULL, IDC_ARROW)) ;
   
  fBlocking = FALSE ;
  fValidBox = TRUE ;
   
  InvalidateRect (hwnd, NULL, TRUE) ;
  }
  return 0 ;
   
  case WM_CHAR :
  if (fBlocking & (wParam == '\x1B')) // i.e., Escape
  {
  DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

  ReleaseCapture () ;
  SetCursor (LoadCursor (NULL, IDC_ARROW)) ;
   
  fBlocking = FALSE ;
  }
  return 0 ;
   
  case WM_PAINT :
  hdc = BeginPaint (hwnd, &ps) ;
   
  if (fValidBox)
  {
  SelectObject (hdc, GetStockObject (BLACK_BRUSH)) ;
  Rectangle (hdc, ptBoxBeg.x, ptBoxBeg.y,
  ptBoxEnd.x, ptBoxEnd.y) ;
  }
   
  if (fBlocking)
  {
  SetROP2 (hdc, R2_NOT) ;
  SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
  Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
  }
   
  EndPaint (hwnd, &ps) ;
  return 0 ;
   
  case WM_DESTROY :
  PostQuitMessage (0) ;
  return 0 ;
  }
  return DefWindowProc (hwnd, message, wParam, lParam) ;
}

当把鼠标拖动到客户区左边或上边时，画出来的矩形不是想象的样子，不知道为什么会出现这个问题，或者怎么解决呢？希望朋友帮忙解释一下，谢谢。


[解决办法]
画出来的矩形大致什么样子的呢？
[解决办法]
判断一下鼠标边界好了简单改了一下，那个2000是我设的比屏幕分辨率要大的值。

C/C++ code

case WM_MOUSEMOVE :        if (fBlocking)        {            SetCursor (LoadCursor (NULL, IDC_CROSS)) ;                        DrawBoxOutline (hwnd, ptBeg, ptEnd) ;                        ptEnd.x = LOWORD (lParam) ;            ptEnd.y = HIWORD (lParam) ;            if(ptEnd.x>2000) ptEnd.x=0;///此二行判断            if(ptEnd.y>2000) ptEnd.y=0;///            DrawBoxOutline (hwnd, ptBeg, ptEnd) ;        }        return 0 ;            case WM_LBUTTONUP :        if (fBlocking)        {            DrawBoxOutline (hwnd, ptBeg, ptEnd) ;                        ptBoxBeg = ptBeg ;            ptBoxEnd.x = LOWORD (lParam) ;            ptBoxEnd.y = HIWORD (lParam) ;            if(ptBoxEnd.x>2000) ptBoxEnd.x=0;////同上            if(ptBoxEnd.y>2000) ptBoxEnd.y=0;////            ReleaseCapture () ;            SetCursor (LoadCursor (NULL, IDC_ARROW)) ;                        fBlocking = FALSE ;            fValidBox = TRUE ;                        InvalidateRect (hwnd, NULL, TRUE) ;        }        return 0 ;