GetTextExtentPoint32(m_hAttribDC, lpszString, nCount, &size)) - mfc

Question

My program is to display some points with their positions(x,y) on a graph. When I use mouse to drag any point, its position will automatically changed. Updated position is implemented following this code(using thread):
m_thread =AfxBeginThread((AFX_THREADPROC)MainThread,this)
UINT CAtwWnd::MainThread(LPVOID pParam)
{
CAtwWnd *pMainDlg = (CAtwWnd*)pParam;
static SChartXYPoint pPoint;
TCHAR strTemp[32]={0,};
while(1)
{
pMainDlg->m_chart.EnableRefresh(false);
pMainDlg->InitGraph1();
wsprintf(strTemp, _T("[%d](%d,%d)"), (int)index,(int)xPoint,(int) yPoint);
pBalloon[index]->SetLabelText(strTemp);
pBalloon[index] = pMainDlg->m_pPointSeries->CreateBalloonLabel(index, strTemp);
pBalloon[index]->SetVisisble(true);
pMainDlg->m_pPointSeries->SetVisible(true);
pMainDlg->m_chart.EnableRefresh(true);
pMainDlg->SetAtwGraphStep(1);
}
return 0;
}
Mean while:
void CChartLabel<PointType>::SetLabelText(const TChartString& strText)
{
m_strLabelText = strText;
m_pParentCtrl->RefreshCtrlAtw();
}
And:
void CChartCtrl::RefreshCtrlAtw()
{
// Window is not created yet, so skip the refresh.
if (!GetSafeHwnd())
return;
if (m_iEnableRefresh < 1)
{
m_bPendingRefresh = true;
return;
}
// Retrieve the client rect and initialize the
// plotting rect
CClientDC dc(this) ;
CRect ClientRect;
GetClientRect(&ClientRect);
m_PlottingRect = ClientRect;
// If the backgroundDC was not created yet, create it (it
// is used to avoid flickering).
if (!m_BackgroundDC.GetSafeHdc() )
{
CBitmap memBitmap;
m_BackgroundDC.CreateCompatibleDC(&dc) ;
memBitmap.CreateCompatibleBitmap(&dc, ClientRect.Width(),ClientRect.Height()) ;
m_BackgroundDC.SelectObject(&memBitmap) ;
}
// Draw the chart background, which is not part of
// the DrawChart function (to avoid a background when
// printing).
DrawBackground(&m_BackgroundDC, ClientRect);
ClientRect.DeflateRect(3,3);
DrawChart(&m_BackgroundDC,ClientRect);
for (int i=0; i<4 ;i++)
{
if (m_pAxes[i])
m_pAxes[i]->UpdateScrollBarPos();
}
Invalidate();
}
when dragging points on graph I gets these errors somtimes: Debug Assertion Failed ( afxwin1.inl, line 639, and 646)
_AFXWIN_INLINE CSize CDC::GetTextExtent(LPCTSTR lpszString, int nCount) const
{
ASSERT(m_hAttribDC != NULL);
SIZE size;
VERIFY(::GetTextExtentPoint32(m_hAttribDC, lpszString, nCount, &size));
return size;
}
_AFXWIN_INLINE CSize CDC::GetTextExtent(const CString& str) const
{
ASSERT(m_hAttribDC != NULL);
SIZE size;
VERIFY(::GetTextExtentPoint32(m_hAttribDC, str, (int)str.GetLength(), &size));
return size;
}
Could you help me to fix this problem? I tried to find some ways to fix but doesn't work. :(

Answer 1

My answer is just a guess, but the reason might be caused by using MFC objects from one thread (the creator) in a second thread. And it is a guess because you didn't told us what the ASSERT say and what VS version you are using.
The problem: When you create some objects in the MFC, the handle values are saved in a map that allows the MFC to find the object only with the handle value.
This handle maps are stored per thread.
Also if a window object stores other objects that are associated with those handle maps, the usage from another thread will fail.
So the answer can be found in the call stack. It tells you who uses such an object. And the object that causes the problem is simply identified by the ASSERT.