GDI: Original DC bitmap changes, and can't restore - winapi

Question

I'm seeing an issue where I could not re-select the original bitmap on a DC, causing a memory leak. The pointer to the original bitmap stayed the same throughout the program, but the data (from CBitmap::GetBitmap) changes from monochrome to something else. I don't know when the bitmap actually changes, but something in the system is causing it.
CBitmap* cMyClass::mpOldBitmap;
CDC cMyClass::mCanvasDc;
CBitmap cMyClass::mCanvasBmp;
void cMyClass::Init()
{
// One-time initialization
CDC* pDc = GetDC();
mCanvasDc.CreateCompatibleDC(pDc);
mCanvasBmp.CreateCompatibleBitmap(pDc, 10, 10);
mpOldBitmap = mCanvasDc.SelectObject(&mCanvasBmp);
ReleaseDC(pDc);
BITMAP bitmap;
mpOldBitmap->GetBitmap(&bitmap); // A monochrome bitmap, as expected.
}
void cMyClass::Recreate(int newW, int newH)
{
// 1. Delete existing bitmap:
if (mpOldBitmap)
{
BITMAP bitmap;
mpOldBitmap->GetBitmap(&bitmap); // This is no longer the monochrome bitmap. It is 8bpp, with random size.
CBitmap* pCurrBmp = mCanvasDc.SelectObject(mpOldBitmap); // This fails (NULL). I can't de-select my bitmap.
mCanvasBmp.DeleteObject(); // This fails too, causing memory leak. Actually, it fails in CE6, but not in Win32. Regardless, both platforms will have a memory leak.
}
// 2. Recreate the bitmap with new size:
{
CDC* pDc = GetDC();
mCanvasBmp.CreateCompatibleBitmap(pDc, newW, newH);
ReleaseDC(pDc);
}
// 3. Finalize
mpOldBitmap = mCanvasDc.SelectObject(&mCanvasBmp);
}
Any known scenarios where this can happen?
Any debugging tips to break when the bitmap data changes?
Note: In code, I mentioned "this fails". I remove the assert on the returned values to make the code readable.
Edit: The solution that I am using to fix it is to use CDC:SaveDC and CDC::RestoreDC instead of stashing the pointer. The memory leak went away, and every GDI call passed. But I am still curious why the original code leaked. The pointer to the default bitmap, as far as I know, should be a default monochrome bitmap that is probably global in GDI world

Answer 1

Let's see the code from OP.
mpOldBitmap = mCanvasDc.SelectObject(mCanvasBmp);
Because mCanvasBmp is a CBitmap object (not a pointer to CBitmap), first is called the HGDIOBJ operator, then CDC::SelectObject(HGDIOBJ) which returns HGDIOBJ and not CBitmap*. This should give a conversion compiler error. If cast the returned value to CBitmap* is also wrong.
The correct way to get rid of problem is to pass a pointer.
mpOldBitmap = mCanvasDc.SelectObject(& mCanvasBmp);
This case will be called CDC::SelectObject(CBitmap* pBitmap) which returns a CBitmap*.
// I hope it's pretty clear. :)