Hello Bob Mills,
Unfortunately there is no way to check whether a handler for an event has been added (other than it firing).
I can suggest several things:
1. If you cannot add the handlers directly to the ListBoxes, add them to one of their visual parents. The handlers from the example should still work. In the example, you can replace the wiring with:
|this.AddHandler(RadDragAndDropManager.DropQueryEvent, new EventHandler<DragDropQueryEventArgs>(OnDropQuery));
|this.AddHandler(RadDragAndDropManager.DragQueryEvent, new EventHandler<DragDropQueryEventArgs>(OnDragQuery));
|this.AddHandler(RadDragAndDropManager.DropInfoEvent, new EventHandler<DragDropEventArgs>(OnDropInfo));
|this.AddHandler(RadDragAndDropManager.DragInfoEvent, new EventHandler<DragDropEventArgs>(OnDragInfo));
...and the drag-drop will still work.
2. Inherit the ListBox and add the handlers in its contstructor. This way you will know that they have been added for sure.
3. Add a debug class handler. Class handlers are handlers that fire at each instance of the given class for the event route. If you add a class handler for FrameworkElement, the handler will be called for each element that the event passes through and you will be able to see if the event does not reach its destination for some reason. To add a class handler for FrameworkElement, you can use this syntax:
| new EventHandler<DragDropQueryEventArgs>(OnDebugDragQuery));
All the best,
the Telerik team