If I udnerstand your scenario correctly I can suggest two things:
If the Telerik DragDrop has not started you can handle the DropQuery event at your application root visual (or on a parent of the diagram) there you can check your global variable and set the e.QueryResult
to disable the Telerik DragDrop.
Handling the DropQuery event at the application root is the last chance to cancel it.
If you need to disable the DragDrop after it has started you can always call the RadDragAndDropManger.CancelDrag
() method to immediately stop it.
Do you think this will help you?
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