Selecting multiple files in File Open dialog

Many professional MDI applications support selecting more than one file in the
File Open dialog. It is easy to add this support to MFC applications.

Opening files (and other document mamangement activities) is implemented in MFC’s CDocMananger
class (see source file docmgr.cpp).
CWinApp contains a member variable m_pDocManager poining to the CDocManager object
to use. If you want to modify the standard MFC behaviour, simply provide your own CDocManager-derived
object.

For adding the “multi open” support, I derived a class CMultiOpenDocManager from CDocManager and
overrode the OnFileOpen function. A new DoPromptFileNames function is a utility function similar to the existing DoPromptFileName function
but capable of returning more than one file name.

class CMultiOpenDocManager : public CDocManager
{
public:
	CMultiOpenDocManager() { }
	virtual void OnFileOpen();
	virtual BOOL DoPromptFileNames(CStringList& fileNames, UINT nIDSTitle, DWORD lFlags, BOOL bOpenFileDialog, CDocTemplate* pTemplate);
};

Using the feature is as simple as adding the following line in your app’s InitInstance function:

	m_pDocManager = new CMultiOpenDocManager;

The implementation of the OnFileOpen function is very similar to the original implementation. I just replaced the call to
DoPromptFileName with a call to my new function DoPromptFileNames, added the OFN_ALLOWMULTISELECT flag
and introduced a loop around the OpenDocumentFile call:

void CMultiOpenDocManager::OnFileOpen()
{
	CStringList newNames;
	if (!DoPromptFileNames(newNames, AFX_IDS_OPENFILE,
	  OFN_HIDEREADONLY | OFN_FILEMUSTEXIST | OFN_ALLOWMULTISELECT, TRUE, NULL))
		return; // open cancelled

	POSITION pos = newNames.GetHeadPosition();
	while (pos)
	{
		CString newName = newNames.GetNext(pos);
		AfxGetApp()->OpenDocumentFile(newName);
	}
}

Now for the DoPromptFileNames function: It is a nearly perfect copy of the original function.
I marked the modifications by a comment. The implementation uses a function AppendFilterSuffix which
is defines as a file local (static) function in docmgr.cpp. You will have to copy the function
into the file where CMultiOpenDocManager::DoPromptFileNames is implemented.

BOOL CMultiOpenDocManager::DoPromptFileNames(CStringList& fileNames, UINT nIDSTitle, DWORD lFlags, BOOL bOpenFileDialog, CDocTemplate* pTemplate)
{
	CFileDialog dlgFile(bOpenFileDialog);

	CString title;
	VERIFY(title.LoadString(nIDSTitle));

	dlgFile.m_ofn.Flags |= lFlags;

	CString strFilter;
	CString strDefault;
	if (pTemplate != NULL)
	{
		ASSERT_VALID(pTemplate);
		AppendFilterSuffix(strFilter, dlgFile.m_ofn, pTemplate, &strDefault);
	}
	else
	{
		// do for all doc template
		POSITION pos = m_templateList.GetHeadPosition();
		BOOL bFirst = TRUE;
		while (pos != NULL)
		{
			CDocTemplate* pTemplate = (CDocTemplate*)m_templateList.GetNext(pos);
			AppendFilterSuffix(strFilter, dlgFile.m_ofn, pTemplate,
				bFirst ? &strDefault : NULL);
			bFirst = FALSE;
		}
	}

	// append the "*.*" all files filter
	CString allFilter;
	VERIFY(allFilter.LoadString(AFX_IDS_ALLFILTER));
	strFilter += allFilter;
	strFilter += (TCHAR)'';   // next string please
#ifndef _MAC
	strFilter += _T("*.*");
#else
	strFilter += _T("****");
#endif
	strFilter += (TCHAR)'';   // last string
	dlgFile.m_ofn.nMaxCustFilter++;

	dlgFile.m_ofn.lpstrFilter = strFilter;
#ifndef _MAC
	dlgFile.m_ofn.lpstrTitle = title;
#else
	dlgFile.m_ofn.lpstrPrompt = title;
#endif
	// --- Begin modifications ---
	// - use a big buffer for the file names 
	// (note that pre-SP2 versions of NT 4.0 will nevertheless
	// truncate the result)
	CString strFileNames;
	dlgFile.m_ofn.lpstrFile = strFileNames.GetBuffer(2048);
	dlgFile.m_ofn.nMaxFile = 2048;

	BOOL bResult = dlgFile.DoModal() == IDOK ? TRUE : FALSE;
	strFileNames.ReleaseBuffer();

	if (!bResult)
		return FALSE; // open cancelled

	// - copy the file names to a string list
	POSITION pos = dlgFile.GetStartPosition();
	while (pos)
	{
		fileNames.AddTail(dlgFile.GetNextPathName(pos));
	}
	return TRUE;
	// --- End modifications ---
}