The implementation of GanttTasks does not provide an order number out of the box. However, you can create a custom GanttTask and include additional properties that suit the needs of your application, in this case a property for the GanttTask order. You can find more information on how to create a custom GanttTask in this help article: http://docs.telerik.com/devtools/wpf/controls/radganttview/features/items/custom-gantttask
Then you can also create a custom GanttDragDropBehavior and override its Drop method, where you can extract the dragged GanttTask, its destination GanttTask and the drop position from the state
method parameter and adjust the order properties accordingly. You can find more information on how to create a custom GanttDragDropBehavior in our Custom Drag and Drop
behaviors help article.
Hope this helps.
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