.NET DependencyInjection via [Service] Attribute

Automatic compile-time service registrations for Microsoft.Extensions.DependencyInjection with no run-time dependencies.

Usage

After installing the nuget package, a new [Service(ServiceLifetime)] attribute will be available to annotate your types:

[Service(ServiceLifetime.Scoped)]
public class MyService : IMyService, IDisposable
{
    public string Message => "Hello World";

    public void Dispose() { }
}

public interface IMyService 
{
    string Message { get; }
}

The ServiceLifetime argument is optional and defaults to ServiceLifetime.Singleton.

NOTE: The attribute is matched by simple name, so you can define your own attribute in your own assembly. It only has to provide a constructor receiving a ServiceLifetime argument.

A source generator will emit (at compile-time) an AddServices extension method for IServiceCollection which you can call from your startup code that sets up your services, like:

var builder = WebApplication.CreateBuilder(args);

// NOTE: **Adds 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();

NOTE: the service is available automatically for the scoped request, because we called the generated AddServices that registers the discovered services.

And that's it. The source generator will discover annotated types in the current project and all its references too. Since the registration code is generated at compile-time, there is no run-time reflection (or dependencies) whatsoever.

Keyed Services

Keyed services are also supported by a separate generic [Service] attribute, like:

public interface INotificationService
{
    string Notify(string message);
}

[Service<string>("sms")]
public class SmsNotificationService : INotificationService
{
    public string Notify(string message) => $"[SMS] {message}";
}

[Service<string>("email")]
public class EmailNotificationService : INotificationService
{
    public string Notify(string message) => $"[Email] {message}";
}

Services that want to consume a specific keyed service can use the [FromKeyedServices(object key)] attribute to specify the key, like:

[Service]
public class SmsService([FromKeyedServices("sms")] INotificationService sms)
{
    public void DoSomething() => sms.Notify("Hello");
}

In this case, when resolving the SmsService from the service provider, the right INotificationService will be injected, based on the key provided.

How It Works

The generated code that implements the registration looks like the following:

static partial class AddServicesExtension
{
    public static IServiceCollection AddServices(this IServiceCollection services)
    {
        services.AddScoped(s => new MyService());
        services.AddScoped<IMyService>(s => s.GetRequiredService<MyService>());
        services.AddScoped<IDisposable>(s => s.GetRequiredService<MyService>());
        
        return services;
    }

Note how the service is registered as scoped with its own type first, and the other two registrations just retrieve the same (according to its defined lifetime). This means the instance is reused and properly registered under all implemented interfaces automatically.

NOTE: you can inspect the generated code by setting EmitCompilerGeneratedFiles=true in your project file and browsing the generated subfolder under obj.

If the service type has dependencies, they will be resolved from the service provider by the implementation factory too, like:

services.AddScoped(s => new MyService(s.GetRequiredService<IMyDependency>(), ...));

MEF Compatibility

Given the (more or less broad?) adoption of MEF attribute (whether .NET MEF, NuGet MEF or VS MEF) in .NET, the generator also supports the [Export] attribute to denote a service (the type argument as well as contract name are ignored, since those aren't supported in the DI container).

In order to specify a singleton (shared) instance in MEF, you have to annotate the type with an extra attribute: [Shared] in NuGet MEF (from System.Composition) or [PartCreationPolicy(CreationPolicy.Shared)] in .NET MEF (from System.ComponentModel.Composition).

Both [Export("contractName")] and [Import("contractName")] are supported and will be used to register and resolve keyed services respectively, meaning you can typically depend on just [Export] and [Import] attributes for all your DI annotations and have them work automatically when composed in the DI container.

Advanced Scenarios

Lazy<T> and Func<T> Dependencies

A Lazy<T> for each interface (and main implementation) is automatically provided too, so you can take a lazy dependency out of the box too. In this case, the lifetime of the dependency T becomes tied to the lifetime of the component taking the lazy dependency, for obvious reasons. The Lazy<T> is merely a lazy resolving of the dependency via the service provider. The lazy itself isn't costly to construct, and since the lifetime of the underlying service, plus the lifetime of the consuming service determine the ultimate lifetime of the lazy, no additional configuration is necessary for it, as it's always registered as a transient component. Generated code looks like the following:

services.AddTransient(s => new Lazy<IMyService>(s.GetRequiredService<MyService>));

A Func<T> is also automatically registered, but it is just a delegate to the actual IServiceProvider.GetRequiredService<T>. Generated code looks like the following:

services.AddTransient<Func<IMyService>>(s => s.GetRequiredService<MyService>);

Repeatedly invoking the function will result in an instance of the required service that depends on the registered lifetime for it. If it was registered as a singleton, for example, you would get the same value every time, just as if you had used a dependency of Lazy<T> instead, but invoking the service provider each time, instead of only once. This makes this pattern more useful for transient services that you intend to use for a short time (and potentially dispose afterwards).

Your Own ServiceAttribute

If you want to declare your own ServiceAttribute and reuse from your projects, so as to avoid taking a (development-only, compile-time only) dependency on this package from your library projects, you can just declare it like so:

[AttributeUsage(AttributeTargets.Class)]
public class ServiceAttribute : Attribute
{
    public ServiceAttribute(ServiceLifetime lifetime = ServiceLifetime.Singleton) { }
}

Likewise for the keyed service version:

[AttributeUsage(AttributeTargets.Class)]
public class ServiceAttribute<TKey> : Attribute
{
    public ServiceAttribute(TKey key, ServiceLifetime lifetime = ServiceLifetime.Singleton) { }
}

NOTE: since the constructor arguments are only used by the source generation to detemine the registration style (and key), but never at run-time, you don't even need to keep it around in a field or property!

With this in place, you only need to add this package to the top-level project that is adding the services to the collection!

The attribute is matched by simple name, so it can exist in any namespace.

If you want to avoid adding the attribute to the project referencing this package, set the $(AddServiceAttribute) to true via MSBuild:

<PropertyGroup>
  <AddServiceAttribute>false</AddServiceAttribute>
</PropertyGroup>

Choose Constructor

If you want to choose a specific constructor to be used for the service implementation factory registration (instead of the default one which will be the one with the most parameters), you can annotate it with [ImportingConstructor] from either NuGet MEF (System.Composition) or .NET MEF (System.ComponentModel.Composition).

Customize Generated Class

You can customize the generated class namespace and name with the following MSBuild properties:

<PropertyGroup>
    <AddServicesNamespace>MyNamespace</AddServicesNamespace>
    <AddServicesClassName>MyExtensions</AddServicesClassName>
</PropertyGroup>

They default to Microsoft.Extensions.DependencyInjection and AddServicesExtension respectively.

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