Dependency Injection

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last updated: 2017-08

Typically, a class constructor is invoked when instantiating an object, and any values that the object needs are passed as arguments to the constructor. This is an example of dependency injection, and specifically is known as constructor injection. The dependencies the object needs are injected into the constructor.

By specifying dependencies as interface types, dependency injection enables decoupling of the concrete types from the code that depends on these types. It generally uses a container that holds a list of registrations and mappings between interfaces and abstract types, and the concrete types that implement or extend these types.

There are also other types of dependency injection, such as property setter injection, and method call injection, but they are less commonly seen. Therefore, this chapter will focus solely on performing constructor injection with a dependency injection container.

Introduction to Dependency Injection

Dependency injection is a specialized version of the Inversion of Control (IoC) pattern, where the concern being inverted is the process of obtaining the required dependency. With dependency injection, another class is responsible for injecting dependencies into an object at runtime. The following code example shows how the ProfileViewModel class is structured when using dependency injection:

public class ProfileViewModel : ViewModelBase  
{  
    private IOrderService _orderService;  

    public ProfileViewModel(IOrderService orderService)  
    {  
        _orderService = orderService;  
    }  
    ...  
}

The ProfileViewModel constructor receives an IOrderService instance as an argument, injected by another class. The only dependency in the ProfileViewModel class is on the interface type. Therefore, the ProfileViewModel class doesn't have any knowledge of the class that's responsible for instantiating the IOrderService object. The class that's responsible for instantiating the IOrderService object, and inserting it into the ProfileViewModel class, is known as the dependency injection container.

Dependency injection containers reduce the coupling between objects by providing a facility to instantiate class instances and manage their lifetime based on the configuration of the container. During the objects creation, the container injects any dependencies that the object requires into it. If those dependencies have not yet been created, the container creates and resolves their dependencies first.

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Note: Dependency injection can also be implemented manually using factories. However, using a container provides additional capabilities such as lifetime management, and registration through assembly scanning.

There are several advantages to using a dependency injection container:

  • A container removes the need for a class to locate its dependencies and manage their lifetimes.
  • A container allows mapping of implemented dependencies without affecting the class.
  • A container facilitates testability by allowing dependencies to be mocked.
  • A container increases maintainability by allowing new classes to be easily added to the app.

In the context of a Xamarin.Forms app that uses MVVM, a dependency injection container will typically be used for registering and resolving view models, and for registering services and injecting them into view models.

There are many dependency injection containers available, with the eShopOnContainers mobile app using Autofac to manage the instantiation of view model and service classes in the app. Autofac facilitates building loosely coupled apps, and provides all of the features commonly found in dependency injection containers, including methods to register type mappings and object instances, resolve objects, manage object lifetimes, and inject dependent objects into constructors of objects that it resolves. For more information about Autofac, see Autofac on readthedocs.io.

In Autofac, the IContainer interface provides the dependency injection container. Figure 3-1 shows the dependencies when using this container, which instantiates an IOrderService object and injects it into the ProfileViewModel class.

Figure 3-1: Dependencies when using dependency injection

At runtime, the container must know which implementation of the IOrderService interface it should instantiate, before it can instantiate a ProfileViewModel object. This involves:

  • The container deciding how to instantiate an object that implements the IOrderService interface. This is known as registration.
  • The container instantiating the object that implements the IOrderService interface, and the ProfileViewModel object. This is known as resolution.

Eventually, the app will finish using the ProfileViewModel object and it will become available for garbage collection. At this point, the garbage collector should dispose of the IOrderService instance if other classes do not share the same instance.

💡 Tip: Write container-agnostic code. Always try to write container-agnostic code to decouple the app from the specific dependency container being used.

Registration

Before dependencies can be injected into an object, the types of the dependencies must first be registered with the container. Registering a type typically involves passing the container an interface and a concrete type that implements the interface.

There are two ways of registering types and objects in the container through code:

  • Register a type or mapping with the container. When required, the container will build an instance of the specified type.
  • Register an existing object in the container as a singleton. When required, the container will return a reference to the existing object.

💡 Tip: Dependency injection containers are not always suitable. Dependency injection introduces additional complexity and requirements that might not be appropriate or useful to small apps. If a class does not have any dependencies, or is not a dependency for other types, it might not make sense to put it in the container. In addition, if a class has a single set of dependencies that are integral to the type and will never change, it might not make sense to put it in the container.

The registration of types that require dependency injection should be performed in a single method in an app, and this method should be invoked early in the app's lifecycle to ensure that the app is aware of the dependencies between its classes. In the eShopOnContainers mobile app this is performed by the ViewModelLocator class, which builds the IContainer object and is the only class in the app that holds a reference to that object. The following code example shows how the eShopOnContainers mobile app declares the IContainer object in the ViewModelLocator class:

private static IContainer _container;

Types and instances are registered in the RegisterDependencies method in the ViewModelLocator class. This is achieved by first creating a ContainerBuilder instance, which is demonstrated in the following code example:

var builder = new ContainerBuilder();

Types and instances are then registered with the ContainerBuilder object, and the following code example demonstrates the most common form of type registration:

builder.RegisterType<RequestProvider>().As<IRequestProvider>();

The RegisterType method shown here maps an interface type to a concrete type. It tells the container to instantiate a RequestProvider object when it instantiates an object that requires an injection of an IRequestProvider through a constructor.

Concrete types can also be registered directly without a mapping from an interface type, as shown in the following code example:

builder.RegisterType<ProfileViewModel>();

When the ProfileViewModel type is resolved, the container will inject its required dependencies.

Autofac also allows instance registration, where the container is responsible for maintaining a reference to a singleton instance of a type. For example, the following code example shows how the eShopOnContainers mobile app registers the concrete type to use when a ProfileViewModel instance requires an IOrderService instance:

builder.RegisterType<OrderService>().As<IOrderService>().SingleInstance();

The RegisterType method shown here maps an interface type to a concrete type. The SingleInstance method configures the registration so that every dependent object receives the same shared instance. Therefore, only a single OrderService instance will exist in the container, which is shared by objects that require an injection of an IOrderService through a constructor.

Instance registration can also be performed with the RegisterInstance method, which is demonstrated in the following code example:

builder.RegisterInstance(new OrderMockService()).As<IOrderService>();

The RegisterInstance method shown here creates a new OrderMockService instance and registers it with the container. Therefore, only a single OrderMockService instance exists in the container, which is shared by objects that require an injection of an IOrderService through a constructor.

Following type and instance registration, the IContainer object must be built, which is demonstrated in the following code example:

_container = builder.Build();

Invoking the Build method on the ContainerBuilder instance builds a new dependency injection container that contains the registrations that have been made.

💡 Tip: Consider an IContainer as being immutable. While Autofac provides an Update method to update registrations in an existing container, calling this method should be avoided where possible. There are risks to modifying a container after it's been built, particularly if the container has been used. For more information, see Consider a Container as Immutable on readthedocs.io.

Resolution

After a type is registered, it can be resolved or injected as a dependency. When a type is being resolved and the container needs to create a new instance, it injects any dependencies into the instance.

Generally, when a type is resolved, one of three things happens:

  1. If the type hasn't been registered, the container throws an exception.
  2. If the type has been registered as a singleton, the container returns the singleton instance. If this is the first time the type is called for, the container creates it if required, and maintains a reference to it.
  3. If the type hasn't been registered as a singleton, the container returns a new instance, and doesn't maintain a reference to it.

The following code example shows how the RequestProvider type that was previously registered with Autofac can be resolved:

var requestProvider = _container.Resolve<IRequestProvider>();

In this example, Autofac is asked to resolve the concrete type for the IRequestProvider type, along with any dependencies. Typically, the Resolve method is called when an instance of a specific type is required. For information about controlling the lifetime of resolved objects, see Managing the Lifetime of Resolved Objects.

The following code example shows how the eShopOnContainers mobile app instantiates view model types and their dependencies:

var viewModel = _container.Resolve(viewModelType);

In this example, Autofac is asked to resolve the view model type for a requested view model, and the container will also resolve any dependencies. When resolving the ProfileViewModel type, the dependency to resolve is an IOrderService object. Therefore, Autofac first constructs an OrderService object and then passes it to the constructor of the ProfileViewModel class. For more information about how the eShopOnContainers mobile app constructs view models and associates them to views, see Automatically Creating a View Model with a View Model Locator.

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Note: Registering and resolving types with a container has a performance cost because of the container's use of reflection for creating each type, especially if dependencies are being reconstructed for each page navigation in the app. If there are many or deep dependencies, the cost of creation can increase significantly.

Managing the Lifetime of Resolved Objects

After registering a type, the default behavior for Autofac is to create a new instance of the registered type each time the type is resolved, or when the dependency mechanism injects instances into other classes. In this scenario, the container doesn't hold a reference to the resolved object. However, when registering an instance, the default behavior for Autofac is to manage the lifetime of the object as a singleton. Therefore, the instance remains in scope while the container is in scope, and is disposed when the container goes out of scope and is garbage collected, or when code explicitly disposes the container.

An Autofac instance scope can be used to specify the singleton behavior for an object that Autofac creates from a registered type. Autofac instance scopes manage the object lifetimes instantiated by the container. The default instance scope for the RegisterType method is the InstancePerDependency scope. However, the SingleInstance scope can be used with the RegisterType method, so that the container creates or returns a singleton instance of a type when calling the Resolve method. The following code example shows how Autofac is instructed to create a singleton instance of the NavigationService class:

builder.RegisterType<NavigationService>().As<INavigationService>().SingleInstance();

The first time that the INavigationService interface is resolved, the container creates a new NavigationService object and maintains a reference to it. On any subsequent resolutions of the INavigationService interface, the container returns a reference to the NavigationService object that was previously created.

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Note: The SingleInstance scope disposes created objects when the container is disposed.

Autofac includes additional instance scopes. For more information, see Instance Scope on readthedocs.io.

Summary

Dependency injection enables decoupling of concrete types from the code that depends on these types. It typically uses a container that holds a list of registrations and mappings between interfaces and abstract types, and the concrete types that implement or extend these types.

Autofac facilitates building loosely coupled apps, and provides all of the features commonly found in dependency injection containers, including methods to register type mappings and object instances, resolve objects, manage object lifetimes, and inject dependent objects into constructors of objects it resolves.

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