Question

I wanted to create an abstract base class for all of my view models, but I’m running into accessibility issues and can’t seem to navigate through the errors. Here’s an example of my code:

public ref class ViewModelBase {
    ...
}

public ref class OtherViewModel : ViewModelBase {
    ...
}

When I define my code as state above, I get the following error:

error C4585: ‘MyNamespace::ViewModelBase’ : A WinRT ‘public ref class’ must either be sealed or derive from an existing unsealed class

If, instead, I make ViewModelBase private, I get this error:

error C3213: base class ‘MyNamespace::ViewModelBase’ is less accessible than ‘OtherViewModel’

This seems like it should be incredibly simple. What am I doing wrong?

Answer 1

What you are attempting is not strictly possible in C++/CX, as in VS 2012 C++/CX does not support public class inheritance for general-purpose scenarios. It is only supported enough to have the XAML scenarios work; the only possible public unsealed types are those found under the Windows::UI::Xaml namespace, and users are not able to author their own public unsealed types.

If you want to have a base class for all your view models, your only options to make all your classes private or to inherit from a base class from Windows::UI::Xaml (such as DependencyObject).

Each of these approaches has drawbacks:

• using DependencyObject as your base makes all your viewmodels STA objects (so they can only be manipulated from the UI thread) in addition to having a lot of overhead that isn’t really necessary.
• Making your classes private means you cannot use the [Bindable] attribute to do databinding, so you would need to have a private databinding implementation. (Your private class would need to implement ICustomPropertyProvider and related interfaces.)