9. Line of Business Scenarios: Dependency Injection

Dependency Injection (DI) is a design pattern to instantiate loosely coupled classes. It's also the fifth statement of the SOLID principles:

One should depend upon abstractions, [not] concretions

The best way to understand it is by a simple example of how to use it. The following code uses DI to implement loose coupling:

public interface IClass2 
{
}

public class Class2 : IClass2
{
}

public class Class1
{
    public readonly IClass2 _class2;
 
    public Class1():this(DependencyFactory.Resolve<IClass2>())
    {
    }
 
    public Class1(IClass2 class2)
    {
        _class2 = class2;
    }
}

This is what we know when we have a look a little closer to this class:

Class1 needs an interface IClass2 to work.
Class1 doesn't know which class is implementing the interface IClass2 and how it was initialized.
If we would want to test Class1, we don't need the Class2 that implements the interface IClass2, we just need a mock of this interface.

Advantages of using DI

There are 2 important reasons to use DI:

Unit Testing: DI enables you to replace complex dependencies, such as databases, with mocked implementations of those dependencies. This allows you to completely isolate the code that is being testing.
Validation/Exception Management: DI allows you to inject additional code between the dependencies. For example, it is possible to inject exception management logic or validation logic, which means that the developer no longer needs to write this logic for every class.

Dependency Injection and ServiceLocator

We are using DI to decoupled our services from the ViewModels and encourage unit testing best practices at the same time. The DI framework we are using in our app is the new Microsoft.Extensions.DependencyInjection created by Microsoft for dotnet core.

Because the application allows opening new windows to permit the user work separately from the rest of the app, we have to be careful when we define the registration of our Services or ViewModels. We have created the ServiceLocator class to solve this problem:

Multiple Service Locators: We need multiple service locators when we open a new window, to avoid re-utilization of the same services in different contexts.

static private readonly ConcurrentDictionary<int, ServiceLocator> _serviceLocators = new ConcurrentDictionary<int, ServiceLocator>();

static public ServiceLocator Current
{
    get
    {
        int currentViewId = ApplicationView.GetForCurrentView().Id;
        return _serviceLocators.GetOrAdd(currentViewId, key => new ServiceLocator());
    }
}

Register Services and ViewModels: This is done in the Configure method. We can divide the registration depending on the way the service will be resolve.

static public void Configure(IServiceCollection serviceCollection)
{
    serviceCollection.AddSingleton<IDataServiceFactory, DataServiceFactory>();
    serviceCollection.AddSingleton<ILookupTables, LookupTables>();
    serviceCollection.AddSingleton<ICustomerService, CustomerService>();
    serviceCollection.AddSingleton<IOrderService, OrderService>();
    ...
    
    serviceCollection.AddScoped<IContextService, ContextService>();
    serviceCollection.AddScoped<INavigationService, NavigationService>();
    serviceCollection.AddScoped<IDialogService, DialogService>();
    serviceCollection.AddScoped<ICommonServices, CommonServices>();

    serviceCollection.AddTransient<ShellViewModel>();
    serviceCollection.AddTransient<MainShellViewModel>();
    ...
    ...

    _rootServiceProvider = serviceCollection.BuildServiceProvider();
}

Resolved as	Services
Singleton: Same service every time we resolve it	Example of these type of services are: IDataServiceFactory, ILookupTables, IMessageService, ILogService, etc
Scoped: A new instance of the service will ve returned if we are in a different scope, i.e. when a new window is opened	Example of these type of services are: IContextService, INavigationService, IDialogService and ICommonServices
Transient: A new version of the service is returned every time we request for it	All the ViewModels of the app are registered as Transient

Software Architecture Line of Business