Drag and Drop - assertion failure

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Hi,

I posted a question yesterday about drag and drop.
I only implemented the drag part (data from my app to other app) and works fine...except one detail.

For exemple. I want to drag text from my app to notepad++.
If notepad++ is visible, every thing works normally.
If notepad++ is minimized, I drag the text from my app, go to the notepad++ icon on explorer taskbar to turn notepad++ visible and I drop the text on it. Works fine, except the time that I will close my application: there is a debug assertion failure in cmdtarg.cpp line:43.

Here is my code:

C++

void CFilesDragDropView::OnInitialUpdate()
{
	CListView::OnInitialUpdate();

	VERIFY( m_DropTarget.Register(this) );
}

DROPEFFECT CFilesDragDropView::OnDragOver(COleDataObject* pDataObject, DWORD dwKeyState, CPoint point) 
{
	if( ! pDataObject->IsDataAvailable( CF_TEXT ) ){
		return DROPEFFECT_NONE;
	}

	DROPEFFECT de = DROPEFFECT_COPY;	
	return de; 
}

void CFilesDragDropView::OnBeginDrag(NMHDR* pNMHDR, LRESULT* pResult) 
{
	HGLOBAL hMem = ::GlobalAlloc(GMEM_ZEROINIT|GMEM_MOVEABLE|GMEM_DDESHARE, 10);
	memcpy( (char*)::GlobalLock(hMem), _T("ola mundo"), 10);
	::GlobalUnlock(hMem);

	DropData.CacheGlobalData( CF_TEXT , hMem );
	DROPEFFECT de = DropData.DoDragDrop(DROPEFFECT_COPY);
	
	*pResult = 0;
}

Can anybody knows why this happens?

Best regards,
Filipe Marques

Posted 26-Sep-13 12:07pm

Filipe Marques

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Jochen Arndt · Accepted Answer · 2013-09-26T21:26:00

I don't know exactly what happens. But I have some notes:

The first two functions are for dropping.

Your code will fail when your project is an Unicode application.

Depending on your Unicode project setting, you must use the matching CF_TEXT or CF_UNICODETEXT format. Note also that there is no implicit conversion between Unicode and ANSI when using Drag & Drop. So the final application should cache strings in both formats.

Your code does not show the definition of the DropData object. It must not be a member of your view class. Define it locally.

You may try this to check if it solves your problem:

void CFilesDragDropView::OnBeginDrag(NMHDR* pNMHDR, LRESULT* pResult) 
{
    LPCTSTR s = _T("ola mundo"); // string to be dragged
    size_t nSize = (_tcslen(s) + 1) * sizeof(TCHAR);
    HGLOBAL hMem = ::GlobalAlloc(GMEM_MOVEABLE, nSize);
    memcpy((char*)::GlobalLock(hMem), s, nSize);
    ::GlobalUnlock(hMem);
    
    COleDataSource * pDataSrc = new COleDataSource; 
#ifdef _UNICODE
    pDataSrc->CacheGlobalData(CF_UNICODETEXT, hMem);
#else
    pDataSrc->CacheGlobalData(CF_TEXT, hMem);
#endif
    DROPEFFECT de = pDataSrc->DoDragDrop(DROPEFFECT_COPY);
    // Must call InternalRelease() when object allocated on the heap.
    pDataSrc->InternalRelease();
    	
    *pResult = 0;
}

The above example allocates the COleDataSource object on the heap. This is the preferred way. When doing so, InternalRelease() must be called afterwards when not calling SetClipboard().