ListView's DragEnter, DragOver, DragDrop event not raised (AllowDrop=True)

Question

Hi all.

This one is very weird:

My app works just fine, but suddenly the damn ListView control's events are not raised any more. It just comes and goes without any clear reason. (Obviously) I've set the AllowDrop property to True and handled the DragEnter, DragOver and DragDrop events as follows:

Private Sub lstApplications_DragDrop(ByVal sender As Object, ByVal e As    System.Windows.Forms.DragEventArgs) Handles lstApplications.DragDrop, Me.DragDrop
    m_fileNames = CType(e.Data.GetData(DataFormats.FileDrop), String())
    mnuType.Show(Cursor.Position, ToolStripDropDownDirection.BelowLeft)
End Sub

Private Sub lstApplications_DragEnter(ByVal sender As Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles lstApplications.DragEnter, Me.DragEnter, lstApplications.DragOver, Me.DragOver
    If chkMode.Checked OrElse Not e.Data.GetDataPresent(DataFormats.FileDrop, True) Then
        e.Effect = DragDropEffects.None
    Else
        e.Effect = DragDropEffects.Copy
    End If
End Sub

It doesn't matter what code I wrote in these two methods because none of the events are raised. Is there anything I'm missing here?

---

I run the same app on another machine and it worked just fine. I then restarted my own machine and everything started working again. I'm not sure, but seems like something was wrong with Windows.

Answer 1

Does the form itself have its AllowDrop property set to true?

Answer 2

Have you tried deleting the method handler stubs, recompiling, then putting them back it and compiling again? I know it sounds wacky but there have been times were I just needed to reset my working copies.

Answer 3

If you're getting intermittent behavior it may be possible that an exception is escaping on of your handlers and an inadvertently disconnecting drag drop. Try adding a blanket Try/Catch block around your code and do a Debug.Fail in the Catch block. That will at least rule out the possibility of an unhandled exception being your problem.

Answer 4

Just remembered we have indeed seen this before, long time ago.

I believe it happens like this:

Drag and drop works fine until some user code throws an exception during a drag and drop operation.

The exception will be eaten; you won't get any error dialog (try it yourself and see). After this point, the drag and drop will stop working.

Answer 5

It's possible that your control's creation is being munged, somehow. Are you certain that the handle exists and that the full set of initialization code has run?

I recently encountered a similar problem where Control.OnHandleCreated wasn't being run due to a mistake I made, and this resulted in all kinds of badness, including exactly what you're describing here.

Answer 6

I've had this problem when running Visual Studio 2008 on Windows 7. VS2008 must run with admin privileges on Windows 7, so I'm running it as a different user. I'm pretty sure this prevents drag 'n' drop working as the application runs fine when run as an app, but drag 'n' drop won't work when being run from Visual Studio.

Answer 7

This post was the answer for me: http://stackoverflow.com/questions/2833709/c-drag-drop-does-not-work-on-windows-7