MFC双缓冲实现画图橡皮条的问题

int CSketchpadView::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
if (CView::OnCreate(lpCreateStruct) == -1)
return -1;
CRect rect;
GetClientRect(&rect);
int nWidth = rect.Width();
int nHeight = rect.Height();

CDC* pDC = GetDC();

m_pMemDCSave->CreateCompatibleDC(NULL);
m_pBitmap.CreateCompatibleBitmap(m_pMemDCSave, nWidth, nHeight);
CBitmap* pOldBit = m_pMemDCSave->SelectObject(&m_pBitmap);
m_pMemDCSave->FillSolidRect(0, 0, nWidth, nHeight, RGB(255, 255, 100));
m_pMemDCSave->BitBlt(0, 0, nWidth, nHeight, pDC, 0, 0, SRCCOPY);

m_pMemDC->CreateCompatibleDC(m_pMemDCSave);
MemBitmap.CreateCompatibleBitmap(m_pMemDC, nWidth, nHeight);
CBitmap* pOldBitmap = m_pMemDC->SelectObject(&MemBitmap);
m_pMemDC->FillSolidRect(0, 0, nWidth, nHeight, RGB(255, 100, 255));
return 0;
}

void CSketchpadView::OnMouseMove(UINT nFlags, CPoint point)