We have created a custom WPF Groupbox control that behaves in a similar manner to the Windows Forms group box.

We are able to place it on a diagram and drag items from a toolbar and drop them into our groupbox. We have also implemented functionality that allows us to drag an item out of the groupbox and drop it onto a diagram.

The thing that we cannot seem to accomplish is selecting a node that has been dropped onto a diagram, then dragging and dropping it into our groupbox. We've tried initiating DragDrop.DoDragDrop() from the Diagram.NodeModifying event handler, as well as from the Diagram.NodeModified handler. But every time that we do so, the diagram's OnDrop() method is called and we can never invoke the drop handling that is built into our control.

Can you provide us with some guidance on how to "bypass" the diagram's OnDrop handling when a node is dragged and dropped over a container control? We can provide a sample project, if necessary.


Thanks,

Greg

Yes, the Groupbox's OnDrop() method is called, and if we set e.Handled = true, then the Diagram's OnDrop() does not get called.

So it seems that I need to modify my original post a bit and say that we don't know exactly where to call the DragDrop.DoDragDrop method.

We have created a new custom node class that inherits from DiagramNode, and in its OnDragOver(DiagramItem) override, we initiate the DragDrop.DoDragDrop, and this seems to work. But we'd like to not have to use a custom node, if possible.

The other issue we see is that we'd like to remove the node that was originally dragged from the diagram, since it is now going to be dropped onto the groupbox. When we do so, however, we are getting a NullReferenceException with the following call stack (truncated):

   at MindFusion.Diagramming.Wpf.DiagramNode.x19a8a761a691d529(Rect xa6236fc5cd405c4e, Point x2f7096dac971d6ec, InteractionState xab29833053004740)
   at MindFusion.Diagramming.Wpf.DiagramNode.x690a7db222c5c9f0(Rect xfcad4c0a9c5890c6, Rect x3bd62873fafa6252, Point x2f7096dac971d6ec, AdjustmentHandles xe72ccf98bea3cb87, InteractionState xab29833053004740)
   at MindFusion.Diagramming.Wpf.DiagramNode.AllowModify(Point current, InteractionState ist)
   at MindFusion.Diagramming.Wpf.InteractionState.ValidateAndSetCursor(Point point, Diagram diagram)
   at MindFusion.Diagramming.Wpf.Behaviors.BehaviorBase.OnMouseMove(Point mousePosition)
   at MindFusion.Diagramming.Wpf.Diagram.OnPreviewMouseMove(MouseEventArgs e)
   at System.Windows.UIElement.OnPreviewMouseMoveThunk(Object sender, MouseEventArgs e)
   at System.Windows.Input.MouseEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.MouseDevice.Synchronize()
   at System.Windows.Input.MouseDevice.ChangeMouseCapture(IInputElement mouseCapture, IMouseInputProvider providerCapture, CaptureMode captureMode, Int32 timestamp)
   at System.Windows.Input.MouseDevice.PreNotifyInput(Object sender, NotifyInputEventArgs e)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.SafeNativeMethods.SafeNativeMethodsPrivate.IntReleaseCapture()
   at MS.Win32.SafeNativeMethods.ReleaseCapture()
   at System.Windows.Interop.HwndMouseInputProvider.System.Windows.Input.IMouseInputProvider.ReleaseMouseCapture()
   at System.Windows.Input.MouseDevice.Capture(IInputElement element, CaptureMode captureMode)
   at System.Windows.UIElement.ReleaseMouseCapture()
   at MindFusion.Diagramming.Wpf.Diagram.x355258a28dc3da25(MouseButtonEventArgs xfbf34718e704c6bc, Point xb30c565aa19aece6)
   at MindFusion.Diagramming.Wpf.Diagram.OnPreviewMouseUp(MouseButtonEventArgs e)
   at System.Windows.UIElement.OnPreviewMouseUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)

 

After some additional work, we have been able to avoid the exception. We begin our drag/drop operation in our custom node's OnDragOver(DiagramItem) override, and the following code was used to avoid the exception:

item.Parent.Behavior = Behavior.DoNothing;

                    if (item.Parent.Interaction != null)
                    {
                        item.Parent.Interaction.SetItemDeleted();
                    }
                    item.Parent.Items.Remove(item);
                    item.Parent.Behavior = Behavior.Modify;
 

Our preference would be to not use a custom node, though. We will try initiating the drag/drop in the NodeStartModifying handler. But by doing so, it seems like we would end up commencing a drag/drop operation every time we modify a node, rather than doing so only when a node is dragged over our groupbox. It also sounds like this approach would prevent us from moving a node, which we need to be able to do. Am I correct on that statement? If so, then I'm not sure that we'd achieve the desired result.

Yes, that's what we are doing: we delete the node from the diagram in the groupbox's OnDrop handler. So it sounds like we are moving in the right direction.

Thank you for the valuable feedback.