vs2005双缓冲绘图下FillRect函数不能通常填充

vs2005双缓冲绘图下FillRect函数不能正常填充？

int APIENTRY _tWinMain(HINSTANCE hInstance,

                     HINSTANCE hPrevInstance,

                     LPTSTR    lpCmdLine,

                     int       nCmdShow)

{

	UNREFERENCED_PARAMETER(hPrevInstance);

	UNREFERENCED_PARAMETER(lpCmdLine);


 	// TODO: 在此放置代码。

	MSG msg;

	HACCEL hAccelTable;


	// 初始化全局字符串

	LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);

	LoadString(hInstance, IDC_WINSAMPLE, szWindowClass, MAX_LOADSTRING);

	MyRegisterClass(hInstance);


	// 执行应用程序初始化:

	if (!InitInstance (hInstance, nCmdShow))

	{

		return FALSE;

	}


    //以下9行为默认代码外添加的

	windowDC = GetDC(hwnd);

	memDC = CreateCompatibleDC(windowDC);

	RECT rect;

	GetClientRect(hwnd, &rect);

	memBMP = CreateCompatibleBitmap(memDC, rect.right - rect.left, rect.bottom - rect.top);

	SelectObject(memDC, memBMP);

	HBRUSH brush = CreateSolidBrush(RGB(255, 0, 0));

	FillRect(memDC, &rect, brush);

	BitBlt(windowDC, 0, 0, rect.right - rect.left, rect.bottom - rect.top, memDC, 0, 0, SRCCOPY);


	hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WINSAMPLE));


	// 主消息循环:

	while (GetMessage(&msg, NULL, 0, 0))

	{

		if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))

		{

			TranslateMessage(&msg);

			DispatchMessage(&msg);

		}

	}


	return (int) msg.wParam;

}

结果是这样的

求解为什么会这样
------解决思路----------------------
memBMP = CreateCompatibleBitmap(windowDC, rect.right - rect.left, rect.bottom - re