When and where to unregister messenger using mvvmlight

Question

When and where to unregister messenger using mvvmlight

I am creating a project with mvvmlight. Use a Messenger message to send messages between virtual machines or virtual machines and a view. Now I wonder when and where to unregister the messenger. Could you help me?

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This is my scene:

There is MainWindow and ChildWindow, ChildWindow (including a ContentControl consisting of one of two different UserControls), change the ContenControl via SwitchButton to ChildWindow. Each window or UserControl corresponds to a ViewModel.

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MainViewModelsends the message MainWindowto open it ChildWindow;
MainWindowopens ChildWindow(first use UserControlView1, do not use UserControlView2) and send a message UserControl1ViewModelto do something;
CancelButton, ChildWindowViewModel ChildWindow ;
ChildWindowViewModel fire UnloadedCommand, ChildWindow UserControlViewmodel -.