DragDrop – DragEnter/DragLeave Events keep firing

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Here is the sequence of events:

  1. Your mouse moves from where you clicked it across the Panel. No events fire.
  2. Your mouse reaches the edge of the Panel and enters the Canvas. The DragEnter event fires on the Canvas.
  3. Your DragEnter handler moves the Panel so that it is now under the mouse. Since the mouse is no longer over the Canvas (it is over the Panel), the DragLeave event fires.
  4. Your mouse moves further, reaching the edge of the Panel again. Steps 2 and 3 repeat.

The faster you move the mouse, the more events you will receive.

The essence of your problem is that drag-drop uses hit testing, and by moving your panel you are defeating the ability of hit testing to see “behind” your panel to know what container it is being dropped in.

The solution is to use your own code for drag and drop handling, which is really not difficult at all. WPF’s hit testing engine is powerful enough to do hit-testing behind the current object but drag-drop doesn’t make use of this functionality. You can use it directly however by using the VisualTreeHelper.HitTest overload that takes a HitTestFilterCallback and a HitTestResultCallback. Just pass it a filter that ignores any hits within the panel being dragged.

I’ve found that for scenarios like you describe, doing drag-drop by handling mouse events is actually easier than using the built in DoDragDrop because you don’t have to deal with the complexity (DataObject, DragDropEffects, QueryContinueDrag, etc). This additional complexity is very important for enabling scenarios of dragging between applications and processes, but does not help you for what you are doing.

Here’s the simple solution:

  1. On MouseDown with left button down, record the position (on MouseLeave erase the position)
  2. On MouseMove with {left button down, position recorded, current mouse position differs by more than delta} set a flag saying drag operation is in progress & capture the mouse
  3. On MouseMove with drag operation in progress, use hit testing to determine where your panel should be (ignoring the panel itself) and adjust its parenting and position accordingly.
  4. On MouseUp with drag operation in progress, release the mouse capture and clear the “drag operation is in progress” flag

Enjoy.

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