I am somewhat new to C# and working through the MVVM tutorial here <https://www.tutorialspoint.com/mvvm/mvvm_hierarchies_and_navigation.htm>

I am having problems with the MainWindowViewModel

```
class MainWindowViewModel : BindableBase {

    public MainWindowViewModel( ) {
        NavCommand = new MyICommand<string>(OnNav);
    }

    private CustomerListViewModel custListViewModel = new CustomerListViewModel( );

    private OrderViewModel orderViewModelModel = new OrderViewModel( );

    private BindableBase _CurrentViewModel;

    public BindableBase CurrentViewModel {
        get { return _CurrentViewModel; }
        set { SetProperty(ref _CurrentViewModel, value); }
    }

    public MyICommand<string> NavCommand { get; private set; }

    private void OnNav(string destination) {

        switch (destination) {
            case "orders":
                //CurrentViewModel = orderViewModelModel;
                break;

            case "customers":
            default:
                //CurrentViewModel = custListViewModel;
                break;
        }
    }
}

```

I can not get the application to build with the above 2 lines setting CurrentViewModel uncommented. I am getting:

* Cannot implicitly convert type 'MVVMHierarchiesDemo.ViewModel.OrderViewModel' to 'MVVMHierarchiesDemo.BindableBase'
* Cannot implicitly convert type 'MVVMHierarchiesDemo.ViewModel.CustomerListViewModel' to 'MVVMHierarchiesDemo.BindableBase'

I did find a typo/bug in the preceding (tutorualspoint) tutorial but don't see anything wrong in the hierarchies tutorial.

Is there a problem that I am not seeing with the example?