GDI+画文本的问题

// CDialogEx::OnPaint();

    CPaintDC dc(this);


     Bitmap membmp(200, 100);
     Graphics grap(&membmp);

     Gdiplus::PointF pf = { 0, 0 };
     Gdiplus::Font myFont(L"新宋体", 16, 0);
     SolidBrush sb(Color(255, 255, 0, 0));
     StringFormat sf;
     grap.DrawString(L"文件", wcslen(L"文件"), &myFont, pf, &sf, &sb);

    Graphics destGrap(dc.GetSafeHdc());
     CachedBitmap cb(&membmp, &destGrap);
     destGrap.DrawCachedBitmap(&cb, 0, 0);

问下上图的“文件”为什么有黑色的部分？怎么去掉？

SetBkColor(dc, 你的窗体背景色);
SetBkMode(dc, TRANSPARENT);