Question

Dragging happens from “source” to “target”. When the source calls DoDragDrop() with allowedEffects as DragDropEffects.Copy, then I’m able to cancel the drop by setting Effects = DragDropEffects.None at the target (in DragOver event).

But the same situation fails when I set my allowedEffects as DragDropEffects.Move.

To reproduce the situation, download the sample from http://jaimersamples.members.winisp.net/samples/dragdrop/drag.zip

Change line.. (to DragDropEffects.Move)

DragDrop.DoDragDrop(this.DragSource, data, DragDropEffects.Copy);

Add line..

void Window1_DragOver(object sender, DragEventArgs args)
{
   args.Effects = DragDropEffects.None;

And also comment out the entire, DragSource_GiveFeedback..

void DragSource_GiveFeedback(object sender, GiveFeedbackEventArgs e)
{
    //System.Diagnostics.Debug.WriteLine("DragSource_GiveFeedback " + e.Effects.ToString());

Is there some kind of bug in the framework, or am I just not able to see something obvious?

Answer 1

After going over your comments and your code again, with some effort I was able to understand and reproduce the problem you are talking about.

What you’re missing is the line:

e.Handled = true;

In your Window1_DragOver event. Add that and it will work the same for both DragDropEffects.Move and DragDropEffects.Copy.

Why it worked for DragDropEffects.Copy without setting e.Handled in the first place is anyone’s guess. Undocumented conditions lead to undocumented behaviour.

I’m going to very strongly recommend that next time you post a code sample containing the minimum possible code to reproduce the problem. I apologize for the original confusion, but nevertheless it was very hard to figure out what was going on in this one.