code: public IStatus validatePluginTransferDrop(
  			IStructuredSelection aDragSelection, Object aDropTarget) {
  		if (!(aDropTarget instanceof IResource)) {
  			return WorkbenchNavigatorPlugin
  					.createStatus(
  							IStatus.INFO,
  							0,
  							WorkbenchNavigatorMessages.DropAdapter_targetMustBeResource,
  							null);
  		}
  		IResource resource = (IResource) aDropTarget;
  		if (!resource.isAccessible()) {
  			return WorkbenchNavigatorPlugin
  					.createErrorStatus(
  							0,
  							WorkbenchNavigatorMessages.DropAdapter_canNotDropIntoClosedProject,
  							null);
  		}
  		IContainer destination = getActualTarget(resource);
  		if (destination.getType() == IResource.ROOT) {
  			return WorkbenchNavigatorPlugin
  					.createErrorStatus(
  							0,
  							WorkbenchNavigatorMessages.DropAdapter_resourcesCanNotBeSiblings,
  							null);
  		}
  
  		IResource[] selectedResources = getSelectedResources(aDragSelection);
  
  		String message = null;
  		if (selectedResources.length == 0) {
  			message = WorkbenchNavigatorMessages.DropAdapter_dropOperationErrorOther;
  		} else {
  			MoveFilesAndFoldersOperation operation;
  
  			operation = new MoveFilesAndFoldersOperation(getShell());
  			message = operation.validateDestination(destination,
  					selectedResources);
  		}
  		if (message != null) {
  			return WorkbenchNavigatorPlugin.createErrorStatus(0, message, null);
  		}
  		return Status.OK_STATUS;
  	}
docs: /*
  (non-Javadoc)
  
  @seeorg.eclipse.ui.navigator.CommonDropAdapterAssistant#
  validatePluginTransferDrop
  (org.eclipse.jface.viewers.IStructuredSelection, java.lang.Object)