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Overview

There are plenty of ASP.NET Core projects out on the internet. All of them have one thing in common. The Startup.cs file. There's also another thing that a lot of these Startup.cs files have in common - they are HUGE.

The Problem

For those unfamiliar with Startup.cs, this is the class that wires up pretty much everything in ASP.NET Core. It's where the request pipeline, dependency injection container, configuration & others get configured. The official docs can be found here. The issue with being responsible for wiring up pretty much everything is that this class tends to grow quite large as more things need to be configured. This problem is compounded with plenty of documentation & tutorials suggesting that, to wire up [insert feature here], all you need to do is drop [insert code snippet here] into Startup.cs and you're off to the races. And the file just keeps on growing...

This causes a few things:

• The Startup class becomes difficult to navigate because there's a lot happening in it
• The Startup class violates the Single Responsibility Principle (SRP) because it no longer has a single reason to change
• It hurts maintainability because there's a high chance of merge conflicts if multiple developers make changes to the Startup class simultaneously

The Problem in Practice

Let's take the following Startup.cs template as a starting point, taken from the .NET Core Web API template (comments removed for brevity):

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

Awesome, clean & simple. For now.

Now let's add our first feature. Any application with reasonable complexity is going to need to store some data somewhere. So, let's add Entity Framework Core & wire it up with SQL Server. As per the docs, we add the following to ConfigureServices:

services.AddDbContext<SchoolContext>(options =>
    options.UseSqlServer(Configuration.GetConnectionString("SchoolContext")));

We also want to be good citizens & document our API so that it can be used by others. So let's use Swashbuckle to add some Open API (Swagger) documentation to our API. So we add the following to ConfigureServices:

services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new OpenApiInfo { Title = "My Awesome API", Version = "v1" });
});

And the following to Configure, inside the env.IsDevelopment() check:

app.UseSwagger();
app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "My Awesome API v1"));

We've also written a couple of custom classes & interfaces which we need to add to the ServiceCollection for Dependency Injection, so let's add them to ConfigureServices:

services.AddScoped<INotificationService, EmailNotificationService>();
services.AddScoped<INotificationService, SmsNotificationService>();
services.AddScoped<INotificationService, PushNotificationService>();

That's enough features for today. Not too many, but enough to start off with. Now our Startup class looks like this:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();

        services.AddDbContext<SchoolContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("SchoolContext")));

        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v1", new OpenApiInfo { Title = "My Awesome API", Version = "v1" });
        });

        services.AddScoped<INotificationService, EmailNotificationService>();
        services.AddScoped<INotificationService, SmsNotificationService>();
        services.AddScoped<INotificationService, PushNotificationService>();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
            app.UseSwagger();
            app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "My Awesome API v1"));
        }

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

It's not too hairy quite yet, but we've only added three features. Imagine what this will look like once we add authentication, authorization, validation, exception handling, logging, caching, customizing Swagger, etc. It can get out of hand for a reasonably-sized application.

The Alternative

The following approach aims to address these issues by refactoring our Startup.cs file so that we can all sleep a little better at night, and we all know that sleep is important.

Let's try to separate some of the Startup class' concerns by refactoring some of them to separate classes. We can use some of the same tricks used by the ASP.NET Core team to accomplish this. For example, if we look at the following line:

services.AddControllers();

What that's doing is configuring all the necessary bits & pieces so that we can use Controllers in an ASP.NET Core Web API, including stuff like Authorization, CORS, Data Annotations, ApiExplorer, etc. But it's all just one line of code. Neat.

That is also an extension method that operates on IServiceCollection. So let's try and write some of our own extension methods which work with IServiceCollection, starting with the database:

internal static class DatabaseServiceCollectionExtensions
{
    public static IServiceCollection AddDatabaseConfiguration(this IServiceCollection services,
        IConfiguration configuration)
    {
        services.AddDbContext<SchoolContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("SchoolContext")));

        return services;
    }
}

Here we have a static class named DatabaseServiceCollectionExtensions because it contains extension methods which operate on IServiceCollection.

There is one static method, AddDatabaseConfiguration which takes the IServiceCollection & IConfiguration as parameters and returns an IServiceCollection. This method then calls the same code we had in Startup to configure Entity Framework Core, and then returns the updated IServiceCollection.

Then we can use it in our Startup class inside ConfigureServices like so:

services.AddDatabaseConfiguration(Configuration);

The Configuration argument is optional for these extension methods, it's just there for the case where the extension method needs something from config in order to perform it's logic, as is the case with the AddDatabaseConfiguration method.

We can then continue this trend for our own application services:

internal static class ApplicationServicesServiceCollectionExtensions
{
    public static IServiceCollection AddApplicationServicesConfiguration(this IServiceCollection services)
    {
        services.AddScoped<INotificationService, EmailNotificationService>();
        services.AddScoped<INotificationService, SmsNotificationService>();
        services.AddScoped<INotificationService, PushNotificationService>();    

        return services;
    }
}

And use it in Startup:

services.AddApplicationServicesConfiguration();

As for Swagger, we needed to make changes to both ConfigureServices and Configure in order to wire it up. Therefore,we'll need two extension methods, one which operates on IServiceCollection as before, and one which operates on IApplicationBuilder for the logic in the Configure method:

internal static class SwaggerExtensions
{
    public static IServiceCollection AddSwaggerConfiguration(this IServiceCollection services,
        IConfiguration configuration)
    {
        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v1", new OpenApiInfo { Title = "My Awesome API", Version = "v1" });
        });

        return services;
    }

    public static IApplicationBuilder UseSwaggerConfiguration(this IApplicationBuilder app)
    {
        app.UseSwagger();

        return app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "My Awesome API v1"));
    }
}

Then we just use them in Startup in their respective methods. In ConfigureServices:

services.AddSwaggerConfiguration();

And in Configure:

app.UseSwaggerConfiguration();

After all this, our Startup class looks like this:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
        services.AddDatabaseConfiguration(Configuration);
        services.AddApplicationServicesConfiguration();
        services.AddSwaggerConfiguration();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
            app.UseSwaggerConfiguration();
        }

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

That's a bit better. 😁

Conclusion

This approach has a few benefits:

• It keeps the Startup class focused by moving configuration logic out to separate classes
• It prevents the Startup class from growing uncontrollably
• It maintains a better separation of concerns such that, when something changes in the configuration of Swagger, for example, that change is isolated to the class which deals with Swagger configuration
• It improves maintainability because it's easier to find & navigate to the class which deals with a specific feature rather than scrolling through the entirety of the Startup class looking for the specific line which needs to be changed

I hope this has proved useful to someone & that it helps you make your applications' code slightly simpler. 😀