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Hot Topic (More than 10 Replies) NullReferenceException when setting DiagramItem.Selected = false (Read 6150 times)
Kris
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NullReferenceException when setting DiagramItem.Selected = false
Jul 4th, 2017 at 1:32pm
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Hi,

In some cases, when I set the Selected property of DiagramItem to false I got the NullReferenceException at: MindFusion.Diagramming.Wpf.Selection.RemoveItem(UIElement item)

any idea what can be null when the item is being removed from selection which cause the nullreference exc ?

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Kris
  
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Slavcho
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #1 - Jul 4th, 2017 at 2:16pm
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Hi,

Is Diagram.ActiveItem null before you change Selected?

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Slavcho
  
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Kris
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #2 - Jul 4th, 2017 at 2:46pm
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Hi,

no, it is not null, I've just debugged it: before setting Selected = false, ActiveItem is not null,
after setting, the exception is thrown and in this moment the ActiveItem is null

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Kris
  
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #3 - Jul 4th, 2017 at 3:38pm
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Does the SelectionChanged event get raised before exception is thrown?
  
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Kris
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #4 - Jul 6th, 2017 at 8:50am
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Looks like yes.
The same exception I got when removing node from the Diagram - exc is thrown in the same place.
The case with removing node is quite random.
Have no idea what can be wrong ...

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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #5 - Jul 6th, 2017 at 9:26am
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I can see RaiseSelectionChanged called on last line of Selection.RemoveItem method. I guess if the event handler is called successfully, then the exception either happens inside it or is somehow caused by how it interacts with the diagram. What does the handler code look like and what is the exception's full call-stack?

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Kris
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #6 - Jul 6th, 2017 at 9:50am
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Hi,

for the node removal case:
   at MindFusion.Diagramming.Wpf.Selection.RemoveItem(UIElement item)
   at MindFusion.Diagramming.Wpf.Diagram.LI(DiagramItem C)
   at MindFusion.Diagramming.Wpf.Commands.RemoveItemCmd.Execute(Boolean undoEnabled)
   at MindFusion.Diagramming.Wpf.UndoManager.V(Command C)
   at MindFusion.Diagramming.Wpf.Diagram.DL(DiagramItem C)
   at MindFusion.Diagramming.Wpf.Diagram.OnRemovingItem(Object sender, ItemEventArgs e)
   at MindFusion.Diagramming.Wpf.ItemCollectionBase`1.RaiseRemoving(DiagramItem item)
   at MindFusion.Diagramming.Wpf.ItemCollectionBase`1.I(T C, Boolean U)
   at ....CommonDiagram.RemoveNode(INode node)

CommonDiagram inherits from Diagram and Remove node just calls: Nodes.Remove(node)
in this case the selectionchanged event is not raised (at least overriden method OnSelectionChanged is not invoked)

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Kris
  
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Slavcho
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #7 - Jul 6th, 2017 at 2:35pm
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Is item.Parent equal to null when the exception's thrown?
  
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Kris
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #8 - Jul 6th, 2017 at 3:30pm
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No, it is not null, it is set to Diagram
  
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #9 - Jul 10th, 2017 at 6:46am
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Are you sure about ActiveItem being non-null then? I can see three lines of code in this method and it's either ActiveItem or Parent. The control supposedly keeps an invariant that ActiveItem is always one of the selected items, but we might be losing it, e.g. if you set ActiveItem from code while processing remove events?
  
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #10 - Jul 11th, 2017 at 2:28pm
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Is there any operation on node or diagram (i.e node.Selected = false) to change the ActiveItem indirectly ?

before: Nodes.Remove -> ActiveItem is not null
after exc is thrown -> ActiveItem is null, Parent is not null

We do not change it directly in the code, so maybe it is changed indirectly, but still have no clue where ...
  
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #11 - Jul 11th, 2017 at 5:20pm
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Selection.RemoveItem sets ActiveItem to either another item from selection, or null if there aren't any selected items left. If there's a single item selected, RemoveItem assumes that's the ActiveItem. There might be some combination of selection changes or deletions from deleted / selected events that breaks that invariant somehow. We'll add null checks there to make sure.
  
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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #12 - Jul 12th, 2017 at 8:17am
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Please check if this build fixes it -
https://mindfusion.eu/_beta/wpfdiag35.zip

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Re: NullReferenceException when setting DiagramItem.Selected = false
Reply #13 - Jul 13th, 2017 at 12:14pm
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Thx,

I worked whole day on this build, looks like solved the problems (and other cases related to this nullrefexc).

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