WM_PAINT中应该用BeginPaint与EndPaint这两个api，它们的功能正是使无效区域恢复（所以WM_PAINT里即使什么都不做，也必须写上BeginPaint与EndPaint）——Delphi里WM_PAINT消息的三个走向都做到了这一点

程序本来是想实现鼠标单击改变背景颜色。可是，程序运行时，为什么没有任何消息触发，背景颜色就一直不断的改变了？WM_PAINT怎么被触发的

#include <windows.h>
#include <stdlib.h>

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

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT ("SetBackgroundColor") ;
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 ("注册失败!"),
szAppName, MB_ICONERROR) ;
return 0 ;
}

hwnd = CreateWindow (szAppName, TEXT ("SetBackgroundColor"),
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 iMsg, WPARAM wParam, LPARAM lParam)
{
static RECT r;
HDC hdc;
HBRUSH hBrush;
switch (iMsg)
{
case WM_PAINT:
hBrush = CreateSolidBrush(RGB(rand()%256,rand()%256,rand()%256));
hdc=GetDC(hwnd);
FillRect (hdc, &r, hBrush) ;
ReleaseDC (hwnd, hdc) ;
DeleteObject (hBrush) ;
return 0;

case WM_SIZE:
GetClientRect(hwnd,&r);
return 0;

case WM_LBUTTONDOWN:
InvalidateRect(hwnd,&r,true);
return 0;

case WM_DESTROY:
PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, iMsg, wParam, lParam) ;
}

回答：这个基础，看一下《windows程序设计》第三章吧

wm_paint是有无效区域的时候产生的消息，所以应首先恢复该区域，才不会一直循环下去

wm_paint中应该用beginpaint与endpaint这两个api，参数是PaintStruct类型，它们的功能正是使无效区域恢复
而不是getDC与releaseDC

invalidaterect也有使区域无效的功能，可用它手动触发wm_paint事件重绘
http://zhidao.baidu.com/question/112410452.html
--------------------------------------------------------------------------------------

对比Delphi代码：

procedure TWinControl.WMPaint(var Message: TWMPaint);
var
DC, MemDC: HDC;
MemBitmap, OldBitmap: HBITMAP;
PS: TPaintStruct;
begin
if not FDoubleBuffered or (Message.DC <> 0) then // 消息里已经自带DC，用这个绘制就可以了
begin
if not (csCustomPaint in ControlState) and (ControlCount = 0) then // 针对Windows自带控件，其包含的子控件数量为0，且没有自绘标记。
inherited // 走向1，主要是针对Windows系统原生控件，微软定义的基本控件，总不会有错，应该内含BeginPaint了吧？
else
PaintHandler(Message); // 走向2，主要是针对Delphi的自绘控件，内含BeginPaint
end
else
begin
DC := GetDC(0); // 走向3，消息里没有自带DC，那么就现取，内含BeginPaint
MemBitmap := CreateCompatibleBitmap(DC, ClientRect.Right, ClientRect.Bottom);
ReleaseDC(0, DC);
MemDC := CreateCompatibleDC(0);
OldBitmap := SelectObject(MemDC, MemBitmap);
try
DC := BeginPaint(Handle, PS);
Perform(WM_ERASEBKGND, MemDC, MemDC);
Message.DC := MemDC;
WMPaint(Message);
Message.DC := 0;
BitBlt(DC, 0, 0, ClientRect.Right, ClientRect.Bottom, MemDC, 0, 0, SRCCOPY);
EndPaint(Handle, PS);
finally
SelectObject(MemDC, OldBitmap);
DeleteDC(MemDC);
DeleteObject(MemBitmap);
end;
end;
end;

procedure TWinControl.PaintHandler(var Message: TWMPaint);
var
I, Clip, SaveIndex: Integer;
DC: HDC;
PS: TPaintStruct;
begin
DC := Message.DC;
if DC = 0 then DC := BeginPaint(Handle, PS);
try
if FControls = nil then PaintWindow(DC) else
begin
SaveIndex := SaveDC(DC);
Clip := SimpleRegion;
for I := 0 to FControls.Count - 1 do
with TControl(FControls[I]) do
if (Visible or (csDesigning in ComponentState) and
not (csNoDesignVisible in ControlStyle)) and
(csOpaque in ControlStyle) then
begin
Clip := ExcludeClipRect(DC, Left, Top, Left + Width, Top + Height);
if Clip = NullRegion then Break;
end;
if Clip <> NullRegion then PaintWindow(DC);
RestoreDC(DC, SaveIndex);
end;
PaintControls(DC, nil);
finally
if Message.DC = 0 then EndPaint(Handle, PS);
end;
end;

就是说，TWinControl无论是否双缓冲，都会调用BeginPaint和EndPaint函数，来消除自绘。整个Controls.pas单元也只有这两处调用BeginPaint。

另外，我没有找到TWinControl的DoubleBuffered被初始化的过程，说明编译器自动把它初始化成了False