Thread: Multithread drawing problem

Hello, I'm trying to draw using CDC objects, each drawing action is done in a thread function. Let's show some code:

Code:

void CMyClass::Draw( CDC *pDCin )
{
	m_pDC = pDCin;
	AfxBeginThread( &CMyClass::draw_threaded, this );
//	CMyClass::draw_threaded( this );
}

UINT __cdecl CMyClass::draw_threaded(LPVOID pparam)
{
        ....
	CMyClass *pcaller = (CMyClass*)pparam;
	CBitmap *pOldBmp;

	if( !pcaller->m_bVisible ) return 0;

	//waiting to be possible to access to object resources
	pcaller->m_csDraw.Lock();           //critical section of this object

	if( pcaller->m_pDC == NULL )
		pcaller->m_pDC = CDC::FromHandle( GetDC( pcaller->m_hwndContainer ));

	CDC DC, *pDC = &DC; //stack allocation

	pDC->CreateCompatibleDC( pcaller->m_pDC );
	pOldBmp = pDC->SelectObject( pcaller->m_pbmp );

	p.CreatePen( PS_SOLID , 1 , COLOR_LABELVALS_BORDER );
	pOldPen = pDC->SelectObject( &p );

	//Drawing rect
	b.CreateSolidBrush( BACK_COLOR );
	pOldBrush = pDC->SelectObject( &b );
	pDC->RoundRect( rpos , roundEllipse );	
	pDC->SelectObject( pOldPen );
	p.DeleteObject();

	//restoring DC objects
	pDC->SetBkMode( prevMode );
	pDC->SelectObject( pOldBrush );
	b.DeleteObject();
	pDC->SetTextColor( oldTextColor );
	pDC->SetBkMode( prevMode );

	//UPDATING SCREEN
	pcaller->m_pDC->BitBlt(	pcaller->m_rectPos.left, 
					pcaller->m_rectPos.top, 
					pcaller->m_rectPos.Width(), 
					pcaller->m_rectPos.Height() , 
					pDC, 0 , 0, SRCCOPY);

	pDC->SelectObject( pOldBmp );
	pDC->DeleteDC();
	pcaller->m_csDraw.Unlock();
	return 0;
}

Now, when I call the function as a thread:

Code:

 AFXBeginThread( &CLinkedLabelVal::draw_threaded, this );

after a first call, the subsequent call causes an assert error:

Code:

MyApp: File afxwin1.inl, Line 598

I took a look to that file and it seems that the m_hDC of the bitmap is null....and this is really strange because I never delete the bitmap, it is a class member of the calling object.

This doesn't happen if I call the draw function in normal way, without using it in a seprarate thread ( using the commented line in the Draw() function):

Code:

CMyClass::draw_threaded( this );

Can somebody give me any hint? Thank you all for help^^

Well - this isn't a good use threads. The resource overhead of creating a new thread (and all the underlying MFC objects) will just slow things down even more.

If you want to speed things up a little, you can cache your memory DC and just keep your bitmap selected to it. The idea is to reduce your "OnDraw" method to just a BitBlt. You may also be able to reduce flicker by handling WM_ERASEBKGND and simply returning FALSE.

gg