MVVM-light There is already a factory registered for INavigationService

Question

I'm trying to adjust my WP8 project from self made MVVM implementation to MVVM Light.

The application compiles without errors, but when I open my MainPage.xaml in Expression Blend, I will get this error:

Class project.Services.INavigationService is already registered. App.xaml

My ViewModelLocator.cs:

    /// <summary>
    /// Initializes a new instance of the ViewModelLocator class.
    /// </summary>
    public ViewModelLocator()
    {
        ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);

        if (ViewModelBase.IsInDesignModeStatic)
        {

        }
        else
        {
            if (!SimpleIoc.Default.IsRegistered<INavigationService>())
            {
                SimpleIoc.Default.Register<INavigationService>(() => new NavigationService());
            }
        }

        SimpleIoc.Default.Register<MainPage>();
        SimpleIoc.Default.Register<SettingsEditViewModel>();
    }

As you can see from my code comment, I've already tried the fix supposed here, but I'm still getting this error in Blend. There is no other place left where I could register the INavigationService, so what could be the problem?

Any ideas? :)

Answer 1

I had the same issue, and this seems to be a Visual Studio issue in combination with XAML-Designer, Static Factories/Locators and Design-Time creation of objects. However: The solutions were the following:

1. Register without a factory (not recommended)

   SimpleIoc.Default.Register<INavigationService>();

2. Or if you want to use a factory, unregister before registering the factory

   SimpleIoc.Default.Unregister<INavigationService>(); SimpleIoc.Default.Register<INavigationService>(() => new NavigationService());

3. Prevent the ViewModelLocator from being created more than once by the designer/Blend by making the constructor static

   static ViewModelLocator() { ... }

The error is cumbersome but could happen in this scenario: You create objects during design-time (the ViewModelLocator within App.xaml probably) and whenever you change something in your Code, the Compiler is triggered and the ViewModelLocator gets re-created without ever unregistering the services. Therefore it will complain that in the factory has already been registered. In theory, when registering classes without factories multiple times, there should be an error as-well.