Finding good information on how to enable drag and drop for MFC applications is hard enough (why?). But just when you think you should have it working, you hit a very solid wall: UIPI (User Interface Privilege Isolation, a variant of UAC, User Account Control). Because of "security" reasons, UIPI disables drag and drop from medium integrity processes (Explorer) to high integrity (aka elevated) processes by filtering out most window messages, including those required for drag and drop. In essence, drag and drop from Explorer to your elevated application does not work.
To waste your time, Microsoft added the API function ChangeWindowMessageFilter that lets specific window messages through (punching a hole in the UIPI "firewall"), but just adding
WM_DROPFILES to the list of allowed messages does not help. Drag and drop uses more than just
WM_DROPFILES. But where is a list of the window messages required for drag and drop? Even the venerable Raymond Chen refused to explain how to make it work. Well, here is how.
Basic Drag and Drop
This assumes you have a dialog-based MFC application. I have implemented this with Visual Studio 2008 SP1.
DragAcceptFiles (); m_oPath.DragAcceptFiles ();
BEGIN_MESSAGE_MAP(<dialog class name>, CDialog)
afx_msg void OnDropFiles(HDROP hDropInfo);
- In your dialog's
OnInitDialog method, enable drag and drop. You can either do this for the entire dialog or a specific control:
ON_WM_DROPFILES() to your message map, e.g.:
- Add an event handler for drop events. In your dialog's header file:
And in the CPP file:
void <dialog class name>::OnDropFiles (HDROP dropInfo)
DWORD nBuffer = 0;
UINT nFilesDropped = DragQueryFile (dropInfo, 0xFFFFFFFF, NULL, 0);
if (nFilesDropped > 0)
nBuffer = DragQueryFile (dropInfo, 0, NULL, 0);
DragQueryFile (dropInfo, 0, sFile.GetBuffer (nBuffer + 1), nBuffer + 1);
UAC and Elevation
Now comes the interesting part. All of the above will work - unless your application runs elevated. As mentioned earlier,
WM_DROPFILES messages are simply filtered out by UIPI. You can re-enable individual messages by calling
ChangeWindowMessageFilter (<message>, MSGFLT_ADD), but enabling
WM_DROPFILES alone is not sufficient. Here is my first attempt to work around that:
for (int i = 0; i <= WM_DROPFILES; i++)
ChangeWindowMessageFilter (i, MSGFLT_ADD);
Hooray! Drag and drop works! But letting all messages through is probably a little bit too much. So I tried to narrow things down.
After some trial and error, I came up with the following list of messages you need to allow through the UIPI filter in order to enable drag and drop:
ChangeWindowMessageFilter (WM_DROPFILES, MSGFLT_ADD);
ChangeWindowMessageFilter (WM_COPYDATA, MSGFLT_ADD);
ChangeWindowMessageFilter (0x0049, MSGFLT_ADD);
Helge Klein is an independent consultant and developer. As a consultant, he has worked in Windows and Citrix projects for various larger German corporations. As a developer, he architected sepago's user profile management product sepagoPROFILE whose successor is now available as Citrix Profile Management. In 2009 Helge received the Citrix Technology Professional (CTP) Award, in 2011 he was nominated a Microsoft Most Valuable Professional (MVP).
Helge's professional interests are focused on Microsoft server technologies, various Citrix products and programming in several languages. He publishes his knowledge in English in his blog at http://helgeklein.com/blog
. Helge can also be found on Twitter as @HelgeKlein
. He has presented on many occasions, e.g. Citrix TechEdge Munich 2009, ice Lingen (2009 and 2011), PubForum (2010 and 2011), Microsoft TechDay Online 2010, Citrix Synergy 2011 and 2012.
Helge is the author of SetACL
, a powerful tool for managing Windows permissions from the command line or from scripts and programs. SetACL is open source and has been downloaded more than 500,000 times. SetACL's modern cousin SetACL Studio
comes with an intuitive graphical user interface and is available for a small fee. Another popular tool, Delprof2
, automates the deletion of user profiles.
Helge lives in Cologne, Germany.