Commands in WPF, Implementation question

Question

I know I've already asked questions on this, but something that's just confusing. I'm doing a tutorial on WPF MVVM pattern, but it seems I cannot get it right, as in the tutorial, it doesn't go into detail on how to implement the ICommand interface. (not sure how correct it really is?!!)

I have the following implementation of ICommand:

class ViewCommand : ICommand
    {
        #region ICommand Members

        private Predicate<object> _canExecute;
        private Predicate<bool> _execute(object param);

        public ViewCommand(Action<object> action)
        {

        }

        public bool CanExecute(object parameter)
        {
            return _canExecute == null ? true : _canExecute(parameter);
        }

        public void Execute(object parameter)
        {
            _execute(parameter);
        }

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

        #endregion
    }

and now this use:

class StartViewModel : ViewModel
    {
        public ViewCommand startViewCommand;

        public ViewCommand StartCommand
        {
            get
            {
                if (startViewCommand == null)
                    startViewCommand = new ViewCommand(
                        param => this.StartApplication()); // StartApplication returns void
                return startViewCommand;
            }
        }

I cannot figure out what my constructor of the ViewCommand class is supposed to do? In the StartViewModel class it's using a lambda expression in the constructor, so there needs to be some delegate, but I'm not sure what and how it's meant to work with Execute and CanExecute. I put a Action<object>, but it's probably wrong...

Lastly, can someone point me to a tutorial on how to implement ICommand that explains it?

Thanks!

Answer 1

Have a look at Josh Smith's tutorial pay attention to relay command, where he implements it.

Answer 2

Your _execute delegate shouldn't be a Predicate<bool>, it should be an Action<object>. Anyway, why don't you use an existing class like Josh Smith's RelayCommand, or MVVM toolkit DelegateCommand ?