If you have worked on applications (in .NET Core) you should know how Dependency Injection (DI) is supported and is applied for your services. This is straight forward and not so hard at all. Just add them to the IServiceCollection in the ConfigureServices() pipeline in the startup.cs file. Considering a N-Tier architrecture, where our Startup.cs is in the Project.Host project, a small example:

// This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddTransient<IService, Service>();
            services.AddScoped<IProcessor, Processor>();
            services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
        }

Now, in the code snippet above, we just add a service, a processor and the IHttpContextAccessor to the DI container. As the application grows and grows, more and more services will be added here and on a long term, this will just bloat up the startup.cs file.

For example, there is a new project added to the solution Project.Reporting which hold services like IPdfReportGeneratorService and a IPdfReportConverterService, and maybe a few more for demonstration purposes. What you would normally do is just add a reference to the reporting project and just add those services to the container. You can see already how this is adding up to the pipeline.

using Solution.Project.Reporting.Services;

// This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddTransient<IPdfReportGeneratorService , PdfReportGeneratorService >();
            services.AddTransient<IPdfReportConverterService, PdfReportConverterService>();
            services.AddTransient<IServiceThis, ServiceThis>();
            services.AddTransient<IServiceThat, ServiceThat>();
            services.AddTransient<IServiceOther, ServiceOther>();

            services.AddTransient<IService, Service>();
            services.AddScoped<IProcessor, Processor>();
            services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
        }

We can clean this up a bit. The Service Collection Extension Pattern is exactly what it says. We add our services in a extension method in its own project. The goal is to keep this pipeline as clear as possible. Let’s do this for the Project.Reporting library in the aplication.

Inside the reporting project, we add a new static class called ReportingServiceCollectionExtensions at the root of the project. Inside we create a extension method where we extend the IServiceCollection and add the services. We also return the services – because the service registration implements the Chain Pattern.

using Microsoft.Extensions.DependencyInjection;

public static class ReportingServiceCollectionExtensions
{
    public static IServiceCollection AddReporting(this IServiceCollection services)
    {
            services.AddTransient<IPdfReportGeneratorService , PdfReportGeneratorService >();
            services.AddTransient<IPdfReportConverterService, PdfReportConverterService>();
            services.AddTransient<IServiceThis, ServiceThis>();
            services.AddTransient<IServiceThat, ServiceThat>();
            services.AddTransient<IServiceOther, ServiceOther>();

        return services;
    }
}

Now we can easily call this extension in our RegisterServices pipeline in the startup.cs file in the host project by calling services.AddReporting();. If you do this for each project/assembly/layer in your solution, this keeps the startup file nice and clean.

using Solution.Project.Reporting.Services;

// This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddReporting();

            services.AddTransient<IService, Service>();
            services.AddScoped<IProcessor, Processor>();
            services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
        }

I hope you understand what the Service Collection Extension Pattern is now and how to implement it. If you have any feedback or questions, do not hesitate to comment.