private void DropBorder_OnPreviewDragLeave(object sender, DragEventArgs e)
 {
     DragDropHelper.SetIsDragOver((DependencyObject)sender, false);
 }
 private void DropBorder_OnDragEnter(object sender, DragEventArgs e)
 {
     DragDropHelper.SetIsDragOver((DependencyObject)sender, true);
 }