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Based on the document: SystemNavigationManagerPreview.CloseRequested Event, it has been mentioned that ClosedRequested events will not be raised for windows that are non-visible. So it is expected behavior that the event will not fire when close it from the taskbar.
And I've made a test about subscribe and unsubscribe the CloseRequested
event. On my side, when I click a button to unsubscribe the event, and then close the app by click the X button, the event will not fire again.
Thank you.
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