CanExecuteChanged event of ICommand

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This event is raised by the command to notify it’s consumers (i.e. Button, MenuItem) that it’s CanExecute property may have changed. So if focus is moved from one TextBox to another, your command may need to be enabled/disabled. This information also needs to be passed to any controls using your command.

In general, this event simply reexposes the CommandManager.RequerySuggested event. From the RoutedCommand class:

public event EventHandler CanExecuteChanged {
    add {
        CommandManager.RequerySuggested += value;
    }
    remove {
        CommandManager.RequerySuggested -= value;
    }
}

The RequerySuggested event is fired quite often, as focus is moved, text selection is changed. This can also be manually raised by calling InvalidateRequerySuggested.

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