When is CanExecute called?

Question

In the demo, I have a button to toggle a bool field isAsking. I create a command which can execute only when isAsking==true.

Once I press T

Answer 1

RoutedCommand contains an event CanExecuteChanged which internally hook to the CommandManager.RequerySuggested event -

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

And CommandManager.RequerySuggested event is raised

whenever changes to the command source are detected by the command manager which is in your case is Window. So, when button is clicked, commandManager raised the RequerySuggested event and hence executed the CanExecute predicate registered for your command.

Also, CommandManager has a static method - InvalidateRequerySuggested which forces the CommandManager to raise the RequerySuggestedEvent. So, you can call that to validate your commands too manually.