RelayCommand

    public class RelayCommand : ICommand
    {
        private readonly Action _execute;
        private readonly Predicate<bool> _canExecute; //изменил Predicate на Predicate<bool>

        public event EventHandler CanExecuteChanged;

        public RelayCommand(Action execute) : this(execute, null) { }

        public RelayCommand(Action execute, Predicate<bool> canExecute) //изменил Predicate на Predicate<bool>
        {
            if (execute == null)
            {
                throw new ArgumentNullException("предикат не должен быть равным нулю");
            }
            _execute = execute;
            _canExecute = canExecute;
        }

        public bool CanExecute(object parameter)
        {
            return _canExecute == null ? true : _canExecute((bool)parameter); //добавил параметр в аргументы с явным преобразованием
        }

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

        public void RaiseCanExecuteChanged()
        {
            var handler = CanExecuteChanged;
            if (handler != null)
            {
                handler(this, EventArgs.Empty);
            }
        }
    }