InvalidCastException when selection changes in a WPF TreeView

To handle the TreeView.SelectedItemChanged event, I have an attached behavior that binds to a command.

Ordinarily, the RoutedPropertyChangedEventArgs.NewValue property contains a reference to one of my view-model objects, and I can then pass this on as the argument to ICommand.CanExecute and ICommand.Execute. I'm using RelayCommand from here, but I've got a RelayCommand<T> that casts to the expected type.

However, in c开发者_如何学编程ertain scenarios, RoutedPropertyChangedEventArgs.NewValue contains a TreeViewItem, displayed as {System.Windows.Controls.TreeViewItem Header:{DisconnectedItem} Items.Count:0} in the debugger.

This causes my RelayCommand<T> to throw an InvalidCastException.

Question: what is this mysterious TreeViewItem and where does it come from?

I'm aware that I can avoid the exception by changing RelayCommand<T>.Execute from using (T)value to using value as T, but I'd like to know what the root cause is.

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A DisconnectedItem is a TreeViewItem that is no longer in your TreeView (i.e. that has been removed from the tree).

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Interesting; do you do anything out of the ordinary with the treeview, as in control templating? Is it the actual built-in treeview or a class that inherits it? It could have something to do with virtualization but it definitely shouldn't happen ordinarily I think.