自绘控件发生了一个奇怪的现象

//这种情况下窗口背景是不发生变化的
	case WM_PAINT:		
		{
			HDC hDCMem;
			hdc = BeginPaint(hWnd, &ps);
			// TODO: Add any drawing code here...
			hDCMem = CreateCompatibleDC(hdc);
			RECT rcWnd = {};
			GetClientRect(_this->m_hWnd, &rcWnd);
			FillRect(hDCMem,&rcWnd,(HBRUSH)(GetStockObject(BLACK_BRUSH)));
			int nWidth = rcWnd.right - rcWnd.left;
			int nHeight = rcWnd.bottom - rcWnd.top;
			BitBlt(hdc,0,0,nWidth,nHeight,hDCMem,0,0,SRCCOPY);
			EndPaint(hWnd, &ps);
		}
		break;

//这种情况下窗口背景是正常的
	case WM_PAINT:		
		{
			hdc = BeginPaint(hWnd, &ps);
			// TODO: Add any drawing code here...
			RECT rcWnd = {};
			GetClientRect(hWnd, &rcWnd);
			FillRect(hdc,&rcWnd,(HBRUSH)(GetStockObject(BLACK_BRUSH)));
			EndPaint(hWnd, &ps);
		}
		break;