DependencyInjection.md

Dependency injection

We use Microsoft.Extensions.Hosting for any IoC related work.

For more documentation on dependency injection, read the references listed at the bottom.

Registering

• Services are registered into IServiceCollection. We can register different services depending on the current configuration defined into the HostBuilderContext.HostingEnvironment.EnvironmentName property.

• We could leverage more hosting extensions under the Microsoft.Extensions.DependencyInjection namespace when adding internal packages. For example, we could have a .AddLocation() extension to include everything IoC related to a location services.

• You can register a singleton service using services.AddSingleton<IService, ServiceImplementation>(). This service will be shared by every user.

  • You don't need to specify any constructor parameters if all dependencies can be resolved automatically. Otherwise, you can use services.AddSingleton<IService>(s => new ServiceImplementation(..)).

• You can register a service that will be created everytime you request it by using services.AddTransient<IService, ServiceImplementation>().

• You can use TryAdd* which will register the service only if there hasn't been a registration of that type yet.

• You can't register named dependencies as this is generally a bad practice.

Resolving

• Dependencies are injected automatically into the constructors of the registered services.

  services.AddSingleton<MyService>();
  services.AddSingleton<MyOtherService>();
  
  public class MyService(MyOtherService myOtherService)
  {
    // Resolving MyService will automatically add MyOtherService here.
  }

• If you can't use constructor injection, you can use IServiceProvider.GetRequiredService<IService>() to resolve a service. This will throw an exception if the type IService is not registered. You can also use IServiceProvider.GetService<IService>() which will return default(IService) if the type is not registered.

• Circular dependencies will not work with this container. If you do have them, you will get the following exception A circular dependency was detected for the service of type.

• You can access the service provider statically using App.Instance.Startup.ServiceProvider (only from the code of the ApplicationTemplate.Shared.Views shared project).

• You can access your services from a view model using this.GetService<MyService>.

• You can also access your services from a view model using the [Inject] attribute.

  // Your class needs to be partial.
  public partial class MyViewModel
  {
    // This property will be automatically assigned in the constructor.
    [Inject] private MyService _service;
  }

  Support of this attribute is done in the ViewModel.cs file.

References

• Understanding Generic Host
• Using dependency injection
• Using Uno.Injectable