Memory DC Help

[MikeB]

I have a static derived class which I want to implement custom drawing in MM_ISOTROPIC mapping mode. For the life of me, I cannot figure out how to do this using a memory dc. I have reviewed and copied several "double buffering" routines, i.e. CMemDC but none of them work when I change the mapping mode.

The following code simply draws a line from the origin (low left corner) to 100,100. It is a simple test. If I use the CPaintDC all is fine, but using a memory dc, nothing shows up.

void DrawCtrl::OnPaint()
{
	CPaintDC dc(this);
	PrepareDc(&dc);

	CDC memDc;
	memDc.CreateCompatibleDC(&dc);
	PrepareDc(&memDc);

	CRect rect;
	dc.GetClipBox(&rect);

	CBitmap bm;
	bm.CreateCompatibleBitmap(&dc, rect.Width(), rect.Height());

	CBitmap* pOld = memDc.SelectObject(&bm);
	CPen wht (PS_SOLID, 1, RGB(255,255,255));
	memDc.SelectObject(&wht);
	memDc.LineTo(100,100);

	dc.BitBlt(rect.left, rect.bottom, rect.Width(), rect.Height(), &memDc, rect.left, rect.bottom, SRCCOPY);
	memDc.SelectObject(pOld);
}
void DrawCtrl::PrepareDc(CDC* pDc)
{
	CRect rect;
	GetClientRect(&rect);

	pDc->FillSolidRect(&rect, RGB(0,0,0));
	pDc->SetMapMode(MM_ISOTROPIC);
	pDc->SetWindowExt(rect.Size());
	pDc->SetViewportExt(rect.Width(), -rect.Height());
	pDc->SetViewportOrg(rect.left, rect.bottom);
}

Any ideas to help me out?

Mike B