CanExecuteChanged event of ICommand

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.

CanExecuteChanged is raised when the CanExecute method of an ICommand gets changed

In some 3rd party libraries, the CanExecuteChanged event also gets raised if the CanExecute parameters raise a PropertyChanged event. For example, MVVM Light Toolkit's RelayCommand raises the CanExecuteChanged event if the CanExecute parameters raise a PropertyChanged event, while Prism's DelegateCommand does not.