Where do I put my login logic in WPF and how do I handle it in MVVM?

Question

I'm currently working on an application that implements the MVVM architecture, where I am trying to do the login logic.
I have a pretty good idea of what the main responsibilities of both INotifyPropertyChanged and ICommand are, and have already implemented them to all my view models that need them.
Here's a short of my code:

I have an ApplicationViewModel.cs that handles the main window ApplicationView.xaml .
The main window has a sidebar menu on the left, and a <ContentControl> that that is binded to the CurrentViewModel property of type ViewModelBase in ApplicationViewModel.cs .
All child view models are contained in a List<ViewModelBase> property. It gets its views from a DataTemplate with the corresponding view model, thus providing me a working navigation system.

So here's what I initially thought of doing:

In a child view model called LoginViewModel.cs , I thought it was logical to do the login logic, as it also handled the view itself. I wrote a model class User.cs with corresponding properties to the database columns. I wrote some classes that would help me get the data from the database to the application.

The view model checks for how many characters are entered and executes the Login once the user has entered 4 characters for the pin code, which instantiates a User object in LoginViewModel.cs

The problem here for me, is that I need the instance of the User to be application wide and therefore probably needs be in the ApplicationViewModel.cs instead.

So, from that, I have 2 questions:

• Is there a better approach to this and a better way handle the login logic?
• If the logic I already made is fine, how do I hand the instance of the User from the child view model LoginViewModel.cs to the parent view model ApplicationViewModel.cs ?

Answer 1

Yes; it wasn't quite natural to come towards this solution when I first started, but you actually can add some kind of 4th layer in MVVM, the Services layer (well, it's more of a pattern than an actual MVVM layer).

Services are single-instance objects that you inject into your viewmodels, and serve exactly that kind of purpose, which is holding and potentially treating data that should circulate between viewmodels.

You will need a dependency injector, a class that will bother with finding the correct service for the classes injected with it.

So you have two concepts to study:

• Services
• Inversion of control/dependency injection

I can't just give you an implementation of a dependency injector, because I personally don't have one.
I'm using a MVVM framework that provides me with one already.

It's basically used this way:

• You create an interface for each of your services; it keeps concerns separated (no coupling between service implementations and viewmodels).
• You register your service implementation by their interfaces (something like myDependencyInjector.Register<IMyService>(typeof(MyService)) at the start of your program
• You add the desired service interfaces to the relevant viewmodels' constructors ( MyViewModel(IMyService myService) )

Answer 2

You could implement custom authentication and authorization by creating classes that derive from the IIdentity and IPrincipal interfaces and override the application thread's default identity. Please refer to the following blog post for more information about how to do this.

Custom authorization in WPF : https://blog.magnusmontin.net/2013/03/24/custom-authorization-in-wpf/ .

You basically create an IPrincipal and use the AppDomain.CurrentDomain.SetThreadPrincipal method to associate this principal with the AppDomain in your application. You can then use the Thread.CurrentPrincipal.Identity.IsAuthenticated property everywhere in your application to determine whether the user is authenticated.

In the code sample, the actual authentication is done by the AuthenticationService class. You could for example implement this one to check the passed in credentials against a remote database. The view model uses this service to perform the authentication.