Question

While copying code for RelayCommand from Josh Smith article I copied following code

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

Then after reading this answer on SO I also copied in my class following code from DelegateCommand class of Prism.

protected void NotifyCanExecuteChanged()
{
    if (CanExecuteChanged != null)
    {
        CanExecuteChanged(this, EventArgs.Empty);
    }
}

But his gives me an error in NotifyCanExecuteChanged method

The event ‘CanExecuteChanged’ can only appear on the left hand side of += or -=

This error is not coming if I remove the add and remove overload from event. Can someone please help me understand the reason behind this?

Answer 1

With a field-like event (which is the name for the simple form without add/remove, then when you do if(CanExecuteChanged != null) or CanExecuteChanged(this, ...), the CanExecuteChanged is referring to the backing field, which is a delegate field of type EventHandler. You can invoke a delegate field. However, that is not the case in your example, because there is no obvious thing to invoke. There certainly isn’t a local field, and the forwarded event (CommandManaged.RequerySuggested) does not intrinsically expose any “invoke” capability.

Basically, for that to work you would need access to an invoke mechanism. Most commonly, I would expect that to take the form of:

CommandManager.OnRequerySuggested();

but if there is a method that invokes this event (and there doesn’t need to be), it could be called anything.

(the On* is a common pattern for a “raise this event” API, doubly-so if it is polymorphic)