Dependency injection using PRISM Xamarin Forms doesn't work

Question

I have a problem with dependency injection in my project. I use PRISM framework in my project and I chose Ioc container when create it. Link -> https://github.com/blackpantific/InstagroomEX In my app.xaml file I associate the class and interface

protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
containerRegistry.RegisterForNavigation();
containerRegistry.RegisterForNavigation<WelcomeView, WelcomeViewModel>();
containerRegistry.RegisterForNavigation<RegistrationView, RegistrationViewModel>();
containerRegistry.RegisterForNavigation<LoginView, LoginViewModel>();

//regestering
        containerRegistry.RegisterSingleton<IValidationService, ValidationService>();
    }

But my page after initializing() doesn't appear at the screen. This is ViewModel constructor

public RegistrationViewModel(INavigationService navigationService, IUserDataService userDataService,
IValidationService validationService) :
base(navigationService)
{
_userDataService = userDataService;
_validationService = validationService;
}

Something wrong here. When I pass to the RegistrationViewModel() constructor another parameters besides INavigationService navigationService the RegistrationView page doesn't not displayed. What am I doing wrong?

You didn't register IUserDataService so PRISM can't find a matching constructor. — Andres Castro, Dec 10 '19 at 19:41
From what I can see from your code, the first page to be displayed shall be `WelcomePage` is that correct? Is `WelcomePage` displayed correctly? — Paul Kertscher, Dec 11 '19 at 13:48
Ireally didn't register another service in Android project. Thank you! — Alexey Vovchenko, Dec 15 '19 at 13:20

Paul Kertscher · Answer 1 · 2019-12-12T06:02:21.077

Assuming that WelcomePage is displayed correctly and RegistrationPage is not, I think that Andres Castro is correct: Prism tries to resolve the IUserDataService which is not possible. In the debug output you should see a message that IUserDataService cannot be resolved.

(Note: The following is based on my experiences with Prism and Unity. This might hold for other Dependency Injection frameworks, but it's possible that other frameworks behave differently.)

Prism relies on a Dependency Injection (DI) framework, for example Microsofts Unity application block. What happens when you try to navigate to RegistrationPage is, that Prism (if you are using ViewModelLocator.AutowireViewModel="True") tries to determine the type of the viewmodel (by convention) and asks the underlying DI framework to resolve this type (i.e. create an instance). For each of the required constructors parameters of this type, then again the DI framework tries to resolve them. If the parameters require concrete types it will then try to resolve those. If the parametery require interface (or abstract) types, the DI framework will look at its registrations and if the types have been registered create instances according to this registrations (which in turn might involve resolving other types).

If - however - the parameter requires an interface type which has not been registered, the DI framework does not know how to handle the situation. It coulld of course assume null, but this might lead to errors that might be way harder to track down, hence it will throw an exception (which is swallowed and Logged by Prism).

Now how can you handle the situation?

You'll need to tell the DI framework how to resolve IUserDataInterface. Since RegistrationPageViewModel does not actually use it, you could register null

containerRegistry.RegisterInstance<IValidationService>(null);

Could be irritating later, though. I'd rather remove the dependency of RegistrationPageViewModel to IUserDataService altogether - for now and add it later when it's actually used - or create a mock/stub that implements IUserDataService and is used as long there is no real implementation. This would be registered with the DI framework as you did with IValidationService.

Dependency injection using PRISM Xamarin Forms doesn't work

1 Answers1

Now how can you handle the situation?