Compile-time automatic dependency injection registrations for .NET

Minimal APIs bring a fresh and clean approach to developing APIs in (ASP).NET 6, with a focus on easy to read and minimal clutter coding.

One area where I’ve always felt the out of the box dependency injection isn’t doing much towards those goals is service registration. Your startup code ends up littered with gazillion builder.Services.AddSingleton<TService, TImplementation>() (and AddTransient and AddScoped as needed). These are annoying to maintain and are a boring repetition of things already stated (and much easier to find) in the type declarations themselves, namely: the implementation type and whatever interfaces it implements.

From the long-time exposure to Visual Studio MEF which is required for most VS extensibility work, I came to deeply appreciate the seamless way you just annotate your types and let the host (VS in that case) figure out what to register, when and how to do it optimally:

[Export(typeof(ICommandHandler))]
class MyCommandHandler 

In most cases, it’s even just [Export] (if you depend on the concrete type elsewhere and not a particular interface) and optionally a [PartCreationPolicy(CreationPolicy.Shared)] to opt-in to singleton behavior.

The fact that the attribute is in the class declaration itself makes it very easy to see how a particular service is registered and its lifetime, make changes as needed, or even remove from the container by just commenting out the attribute.

Run-time vs compile-time Registrations

In the past, I’ve dealt with this by simply doing an assembly scanning at app startup and just invoking the registration methods, but this can have a non-trivial impact on app startup time, especially as the project grows and starts having more and more project dependencies that also need to be scanned.

Luckily, Roslyn provides a high-performance mechanism to solve this, called incremental generators, which can scale to large solutions with many assemblies and types so that build time isn’t impacted severely while still having great run-time caracteristics since you perform all expensive evaluation at compile-time.

Automatic compile-time codegen for registrations

So I put the idea to test and created an attributed dependency injection generation nuget package that allows you to simply annotate types like you would in MEF:

[Service]
public class MyService : IMyService
{
}

Optionally specifying the desired lifetime (which is otherwise Singleton):

[Service(ServiceLifetime.Scoped)]
public class MyService : IMyService
{
}

With the package installed, code will be generated automatically for the services in the current project or any dependencies, which is essencially an extension method on IServiceCollection so you can register all discovered services:

var builder = WebApplication.CreateBuilder(args);

// Add discovered services to the container.
builder.Services.AddServices();
// ...

var app = builder.Build();

// Configure the HTTP request pipeline.
app.MapGet("/", (IMyService service) => service.Message);

// ...
app.Run();

How it works

The extension method looks like the following:

static partial class AddServicesExtension
{
    public static IServiceCollection AddServices(this IServiceCollection services)
    {
        AddScopedServices(services);
        AddSingletonServices(services);
        AddTransientServices(services);

        return services;
    }

    static partial void AddScopedServices(IServiceCollection services);
        
    static partial void AddSingletonServices(IServiceCollection services);

    static partial void AddTransientServices(IServiceCollection services);
}

Each partial method is then generated in turn and populated with registration calls for each service with the respective lifetime, such as:

static partial class AddServicesExtension
{
    static partial void AddScopedServices(IServiceCollection services)
    {
        services.AddScoped<MyService>();
        services.AddScoped<IMyService>(s => s.GetRequiredService<MyService>());
    }
}

Note that instead of doing services.AddScoped<IMyService, MyService>(), the generated code registers the implementation first, and then registers the interface to resolve it. This allows proper behavior when the service implements more than one interface: resolving the scoped service using any of the implemented interfaces will result in the same scoped instance being created/retrieved. This is important since the generated code will register your services with all the interfaces they implement, so you don’t need anything except the [Service] attribute :). This is the most intuitive thing for me, so I made it a built-in convention.

If you instead do multiple <TService, TImplementation> registrations, each registration gets its own lifetime in the service collection (somewhat surprisingly, since the TImplementation is only one…).

Installing

Just add a package reference to the main project (doing the service registrations) and you will automatically get everything you need.

<PackageReference Include="Devlooped.Extensions.DependencyInjection.Attributed" Version="*" />

or via CLI:

dotnet add package Devlooped.Extensions.DependencyInjection.Attributed

There are some advanced use cases documented in the package readme too that may be helpful too.

Enjoy!