/// This is our MainViewModel that is tied to the MainWindow via the 
    /// ViewModelLocator class.
    /// </summary>
    public class MainViewModel : ViewModelBase
    {
        /// <summary>
        /// Static instance of one of the ViewModels.
        /// </summary>
        private static readonly SecondViewModel SecondViewModel = new SecondViewModel();
        /// <summary>
        /// Static instance of one of the ViewModels.
        /// </summary>
        private static readonly FirstViewModel FirstViewModel = new FirstViewModel();
        /// <summary>
        /// The current view.
        /// </summary>
        private ViewModelBase _currentViewModel;
        /// <summary>
        /// Default constructor.  We set the initial view-model to 'FirstViewModel'.
        /// We also associate the commands with their execution actions.
        /// </summary>
        public MainViewModel()
        {
            CurrentViewModel = FirstViewModel;
            FirstViewCommand = new RelayCommand(ExecuteFirstViewCommand);
            SecondViewCommand = new RelayCommand(ExecuteSecondViewCommand);
        }
        /// <summary>
        /// The CurrentView property.  The setter is private since only this 
        /// class can change the view via a command.  If the View is changed,
        /// we need to raise a property changed event (via INPC).
        /// </summary>
        public ViewModelBase CurrentViewModel
        {
            get { return _currentViewModel; }
            set
            {
                if (_currentViewModel == value)
                    return;
                _currentViewModel = value;
                RaisePropertyChanged("CurrentViewModel");
            }
        }
        /// <summary>
        /// Simple property to hold the 'FirstViewCommand' - when executed
        /// it will change the current view to the 'FirstView'
        /// </summary>
        public ICommand FirstViewCommand { get; private set; }
        /// <summary>
        /// Simple property to hold the 'SecondViewCommand' - when executed
        /// it will change the current view to the 'SecondView'
        /// </summary>
        public ICommand SecondViewCommand { get; private set; }
        /// <summary>
        /// Set the CurrentViewModel to 'FirstViewModel'
        /// </summary>
        private void ExecuteFirstViewCommand()
        {
            CurrentViewModel = FirstViewModel;
        }
        /// <summary>
        /// Set the CurrentViewModel to 'SecondViewModel'
        /// </summary>
        private void ExecuteSecondViewCommand()
        {
            CurrentViewModel = SecondViewModel;
        }
    }