C# Drag drop does not work on windows 7

Question

I have had a project for quite a while using C# winforms. I implemented a drag-drop function before windows 7 was released. Worked like a charm. However, when using windows 7 it does not work. The event doesn't even get triggered.

AllowDrop is set to true. When subscribing to DragEnter it does not get called in windows 7 (not sure about vista). But on XP it works all the way. The program is run with administritave priviliges.

Is there any difference in the drag drop in windows 7 vs xp? Don't know if it's relevant, but I'm using x64

Answer 1

The source and target processes need to have compatible security levels\\ privileges. For example, if your source is Explorer and it is running with user level privileges, but your target application is running with administrator (elevated) level permission, you will not be able to drag&drop as this is seen as a security issue as the target is running with a higher level of privileges.

Answer 2

It is called UIPI, User Interface Privilege Isolation. Designed to prevent input injection exploits from programs that run with restricted privileges. It can be disabled, you'll need to do this:

• Modify the manifest, set the uiAccess attribute for the <requestedExecutionLevel> element to true.
• Store your program's EXE in a subdirectory of c:\\windows or c:\\program files
• Sign your EXE with a certificate from an valid code signing authority

Never actually tried this, ymmv.

Answer 3

It may be unrelated, but I was having whacky Drag-n-Drop issues with Windows 8. It would work for a while and then stop working. I couldn't drag-n-drop between Explorer, in an editor, etc. It turns out that when I interacted with a Hyper-V VM running windows 7, my drag-n-drop ability was altered (or perhaps it was the ctrl-alt-end keystroke to simulate crtl-alt-delete to Hyper-V). In any event, one the issue occurred, the following resolved it:

I found the resolution to my problem here: Fix Drag Drop Functionality Not Working In Windows 7 Vista XP

Basically the solution was:

Left-Click on a file in explorer, and while holding down the mouse button, press [Esc] then [Ctrl], then release the mouse button. I have to assume that this is resetting some accessibility setting or something.

Answer 4

From your application, call ChangeWindowMessageFilter with the following values to allow dragging and dropping to/from your elevated application and non-elevated applications like Explorer:

ChangeWindowMessageFilter (WM_DROPFILES, MSGFLT_ADD);
ChangeWindowMessageFilter (WM_COPYDATA, MSGFLT_ADD);
ChangeWindowMessageFilter (0x0049, MSGFLT_ADD);

Answer 5

A Minor addition to dmex's post. The following defines the variables and the constant.

private const uint WM_DROPFILES = 0x233;
private const uint WM_COPYDATA = 0x004A;
private const uint WM_COPYGLOBALDATA = 0x0049;
private const uint MSGFLT_ADD = 1;

Also, you may want to consider using ChangeWindowMessageFilterEx if you're application is on Windows 7. I also believe that OLE drag and drop may not use Windows messaging. So it wouldn't effect that at all.

Answer 6

I had same problem when debugging a desktop C# application from VS 2015 and Windows 7 x64 as SO It was due to the administrator permission applied over the shortcut of my VS IDE in the desktop (see screenshot). Unchecking this option Drag & Drop events raise properly.

Thanks to TimLloyd for his help.