I’ve decided to write a tutorial on how to accomplish dependency injection in C# .NET, because the Microsoft documentation of dependency injection libraries is unfortunately way too sparse, and the Microsoft dependency injection tutorial is a little convoluted and complicated.

Fortunately, C# .NET’s implementation of dependency injection is pretty straightforward. In my opinion, it’s way more straightforward than the implementation provided by Java’s Spring Framework. If you understand the basics of the dependency injection concept but haven’t yet tried it out in practice, C# .NET could be your best bet.

Dependency injection recap

Here’s a quick recap on what dependency injection entails. If you want more detail, this article I wrote may be helpful.

In general, whether dependency injection is in play or not, classes may specify types- called dependencies– that they have has-a relationships with.

In dependency injection, instances of classes are not responsible for creating instances of their dependencies. Instead, a managing container maintains has-a relationships with instances of the classes, and the user specifies to the container which implementations of the dependencies they want to use by calling one of the container’s methods, or by writing so-called “configuration code” that is interpreted by the container. At runtime, the container “injects” these implementations into the class instances.

Why use dependency injection? The main point is to separate interface from implementation. Why is this important? I suggest you read the linked article above for more details.

.NET dependency injection terminology

The first thing to be aware of when learning dependency injection in C# .NET is that Microsoft uses some alternative terminology when discussing dependency injection concepts. If you want to be able to understand the Microsoft documentation, you need to be aware of this terminology. So, here’s some vocabulary:

ServiceDescriptor

The ServiceDescriptor class is what represents a service (recall that “service” means “dependency”). The most down-to-earth constructor of ServiceDescriptor is as follows:

public ServiceDescriptor (Type serviceType, Type implementationType, ServiceLifetime lifetime)

So, we see that in C# .NET, a service essentially wraps the type of the dependency, the type of the preferred implementation for said dependency, and the “lifetime” of the dependency.

ServiceLifetime

In my opinion, “lifetime” should really be called “instantiation multiplicity”, since the value of lifetime in the above constructor determines whether or not the management container is to create multiple instances of the dependency, and, if so, how to do so.

Specifically, ServiceLifetime is an enum that can take on the value Singleton, Transient, or Scoped.

• Singleton indicates that the management container (which we have not seen yet) will ensure that only one instance of the service will be created throughout the program lifetime. All class instances which depend on the service will share the same service.

• Transient indicates that the management container will ensure that a new instance of the dependency will be created whenever a different class instance needs it.

• The meaning of Scoped is a little complicated for a first pass at dependency injection in C# .NET. If you want to learn about it, read this.

ServiceDescriptor properties

You already saw the ServiceDescriptor constructor, which is what’s most important in regards to understanding ServiceDescriptor. For a bit more detail, here are the public properties that are wrapped by the ServiceDescriptor:

public Func<IServiceProvider,object>? ImplementationFactory { get; } // a factory method that stores instructions on how to build an instance of the implementation type
public object? ImplementationInstance { get; }
public Type? ImplementationType { get; } // type of the wrapped instance, ImplementationInstance
public ServiceLifetime Lifetime { get; }
public Type ServiceType { get; } // type of the wrapped interface

Some of the above may be confusing, so here are some clarifying notes:

• Func<T1, T2> represents a function that takes an argument of type T1 as input and returns a type T2 instance as output. Thus, the ImplementationFactory property is a function that takes an IServiceProvider as input and returns an instance of the implementation as output. ImplementationFactory can be thought of as wrapping instructions for how to create an instance of the implementation instance.

• For any type T, the expression T? is shorthand for Nullable<T>, which represents a nullable version of the type T. A type is called nullable if compiler errors are not thrown when a null value of said type is attempted to be used. For more context on Nullable<T> is, see the below appendix.

Registering services (ServiceDescriptors) with IServiceCollection

So far, we know how to represent services (dependencies) as ServiceDescriptors. We’ll now learn how to create a managing container and how to register our services with said container.

An instance of type IServiceCollection is what will represent our managing container. From its interface definition, we can see that IServiceCollection a collection of  ServiceDescriptors. (So, IServiceCollection is interpreted as “I{ServiceCollection}“, which means “interface to a collection of services”, not “{IService}Collection“, which would mean “collection of interfaces to services”!).

using Microsoft.Extensions.DependencyInjection;
using System.Collections.Generic;​
public interface IServiceCollection : ICollection<ServiceDescriptor>, IEnumerable<ServiceDescriptor>, IList<ServiceDescriptor> { }

Microsoft provides an implementation of IServiceCollection for us- the ServiceCollection class from the Microsoft.Extensions.DependencyInjection namespace- so we don’t have to take care of the implementation ourselves.

Service registration via extension methods to IServiceCollection

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In order to store services in an IServiceCollection, we need to enable access to some extension methods to IServiceCollection.

(An extension method is an instance method of a class that is added to the class after the class is defined. Confusingly, you can add extension methods, which are non-abstract methods, to an interface. To learn more about extension methods, see the below appendix).

To obtain access to the extension methods we need, just include a using Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions statement at the top of the file.

Some important extension methods (with the parameter for the extended class, IServiceCollection, omitted) added by ServiceCollectionServiceExtensions are:

AddSingleton(Type serviceType, Type implementationType);
AddSingleton(Type serviceType); // is the above with implementationType = serviceType

Being extension methods to IServiceCollection, these methods are invoked on an instance of type IServiceCollection in the same way that usual instance methods are. For example, if services has type IServiceCollection, then we would call the above methods by writing

services.AddSingleton(serviceType, implementationType);
services.AddSingleton(serviceType);

You can probably surmise that these two methods add a ServiceDescriptor of lifetime Singleton that has the specified serviceType and implementationType to the IServiceCollection.

There are also versions of the above methods for which certain combinations of the parameters are held fixed:

AddSingleton<TService,TImplementation>()
AddSingleton<TService>(); // is the above with TImplementation = TService
AddSingleton<TService,TImplementation>(Func<IServiceProvider,TImplementation> implementationFactory)

And, of course, for every method whose name is AddSingleton, there will be corresponding methods with names of AddTransient and AddScoped that perform the same task for services of Transient and Scoped lifetimes, respectively.

Though, these two versions of AddSingleton, which specify the instance that is to be wrapped by the singleton service, don’t have AddTransient or AddScoped counterparts, because it wouldn’t make sense to specify only a single instance to AddTransient or AddScoped:

AddSingleton(Type serviceType, object implementationInstance);
AddSingleton<TService>(TService instance);

Resolving services at runtime with IServiceProvider

At this point, we know how to we know how to represent services (dependencies) as ServiceDescriptors, how to create how to create a managing container, and how to register our services with the container. The last item we need to address is that of configuring the resolving of services at runtime (i.e. configuring the injection of dependencies at runtime).

Let’s suppose that services is an IServiceCollection (i.e. a managing container) that contains some ServiceDescriptors (i.e. “services”, or dependencies).

To grab services from a managing container named services at runtime, we will first obtain an instance of type IServiceProvider from the managing container* by storing the return value of services.BuildServiceProvider() . Then, we grab a particular service by using the single abstract method specified in the IServiceProvider interface:

public object? GetService(Type serviceType)

* services.BuildServiceProvider returns an instance of Microsoft.Extensions.DependencyInjection.ServiceProvider, which implements IServiceProvider.

Somewhat random: creating IServiceCollections by using IServiceProviders

This section is pretty optional.

If you already have one IServiceCollection instance and corresponding IServiceProvider, and you want to create another IServiceCollection instance by making use of dependencies stored in the first IServiceCollection instance, you can use these extension methods to IServiceCollection:

AddSingleton(Type serviceType, Func<IServiceProvider, object> factory);
AddSingleton<TService,TImplementation>(Func<IServiceProvider,TImplementation> implementationFactory);
AddSingleton<TService>(Func<IServiceProvider, TService> implementationFactory); // is the above with TImplementation = TService

Appendix

This appendix documents some less commonly known language features of the C# language.

? and nullable reference types

A non-nullable type is a type for which compiler errors are thrown when a variable of that type with null value is attempted to be used . Contrastingly, a nullable type is a type for which compiler errors are not thrown in said situation. You can still get runtime errors with nullable types, of course! The whole point of non-nullable types is to avoid runtime errors by catching them at compilation.

According to the Microsoft documentation, all reference types were nullable prior to C# 8.0. Nowadays (i.e. after C# 8.0), all reference types are non-nullable by default.

You can still use nullable types if you really want, though. For any type T, the typeNullable<T> is nullable. ?T is shorthand for Nullable<T>.

Extension methods

In C#, it is possible to define instance methods outside of the corresponding class definition. Methods defined in this way are called extension methods.

Extension methods must be defined in a static class, and must use the this keyword in the following way:

public class Cls { ... }

​public static class Extension
{
     public static int extensionMethod1(this Cls cls)
     { int someValue = 0; return someValue; }       
     
     public static int extensionMethod2(this Cls cls, int arg)
     { int someValue = 0; return someValue; } 
}

Extension methods are called in the same way as regular instance methods: to call the above defined extension methods on an instance cls of Cls, you would write cls.extensionMethod() or cls.extensionMethod2(arg), respectively.

Extension methods to interfaces

Somewhat confusingly, it is possible to define extension methods- in exactly the same way as above- for interfaces. To me this possibility runs contradictory to the intent of “interface”- interfaces are not supposed to be associated with actual implementations of methods. But you can do it. It is also in fact impossible to add something like an “abstract extension method” to an interface. The C# standard library unfortunately makes much use of implementing interfaces via extension methods. Oh well.

References

I referenced the following two articles in developing my understanding of IServiceCollection: (1), (2).