如何在内存中画好图，再在屏幕上显示出来？

下面的代码没有达到预期的效果，请高手指点一二，谢谢！
void CSaveBmpView::OnDraw(CDC* pDC)
{
CSaveBmpDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);

CRect rect;
GetClientRect(&rect);

CDC dcMem;
dcMem.CreateCompatibleDC(pDC);

BITMAPINFO bi;
memset(&bi,0,sizeof(bi));
bi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bi.bmiHeader.biWidth = rect.Width();
bi.bmiHeader.biHeight = rect.Height();
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 24;
bi.bmiHeader.biCompression = BI_RGB;

UINT *bits;
HBITMAP hBitmap =
::CreateDIBSection(dcMem.m_hDC,&bi,DIB_RGB_COLORS,(void**)&bits,NULL,0);

::SelectObject(dcMem,hBitmap);

//填充一个四边形
int xLeft,yTop,xRight,yBottom;
xLeft = rect.left + 100;
yTop = rect.top + 100;
xRight = rect.right - 100;
yBottom = rect.bottom - 100;

dcMem.MoveTo(xLeft,yTop);
dcMem.LineTo(xRight,yTop+50);
dcMem.LineTo(xRight-50,yBottom);
dcMem.LineTo(xLeft,yBottom-50);
dcMem.LineTo(xLeft,yTop);

CBrush brush;
brush.CreateSolidBrush(RGB(0,255,0));
CBrush* pOldBrush = dcMem.SelectObject(&brush);

dcMem.FloodFill(xLeft+10,yTop+50,RGB(0,0,0));

dcMem.SelectObject(pOldBrush);
//End fill!

pDC->BitBlt(0,0,rect.Width(),rect.Height(),&dcMem,rect.left,rect.top,
SRCCOPY);

SaveBmpToFile("Temp.bmp",&bi,bits);
}

int CSaveBmpView::SaveBmpToFile(const char *filename, BITMAPINFO *info, UINT *bits)
{
FILE* fp;
UINT size, //Size of file
infosize, //Size of bitmap info
bitsize; //Size of bitmap pixels
BITMAPFILEHEADER header;
if((fp = fopen(filename,"wb")) == NULL)
return -1;
if(info->bmiHeader.biSizeImage == 0)
bitsize = (info->bmiHeader.biWidth *
info->bmiHeader.biBitCount+7)/8*
abs(info->bmiHeader.biHeight);
else
bitsize = info->bmiHeader.biSizeImage;

infosize = sizeof(BITMAPINFO);
size = sizeof(BITMAPFILEHEADER)+infosize+bitsize;

header.bfType = 'MB';
header.bfSize = size;
header.bfReserved1 = 0;
header.bfReserved2 = 0;
header.bfOffBits = sizeof(BITMAPFILEHEADER)+infosize;

if(fwrite(&header,1,sizeof(BITMAPFILEHEADER),fp) <
sizeof(BITMAPFILEHEADER))
{
fclose(fp);
return -1;
}

if(fwrite(info,1,infosize,fp) < infosize)
{
fclose(fp);
return -1;
}

if(fwrite(bits,1,bitsize,fp) < bitsize)
{
fclose(fp);
return -1;
}

fclose(fp);
return 1;
}