ASP.NET Core Startup Class

In ASP.NET Core, the Startup class is where you define how your application is built and how it will handle requests. The two key methods — ConfigureServices and Configure — serve different but complementary purposes in the app’s startup lifecycle.

1️⃣ ConfigureServices(IServiceCollection services)

Purpose: To register application services and configure Dependency Injection (DI).

What happens here:

• You add framework services (e.g., MVC, Razor Pages, Identity, EF Core).
• You register your own application services (e.g., repositories, business logic classes).
• You configure options and bind configuration sections.

Key points:

• Runs before Configure.
• Populates the DI container so services can be injected into controllers, middleware, etc.
• Uses services.Add... methods.

Example:

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services
    services.AddControllersWithViews();
    
    // Configure EF Core
    services.AddDbContext<AppDbContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

    // Register custom services
    services.AddScoped<IEmailService, EmailService>();

    // Configure options
    services.Configure<MySettings>(Configuration.GetSection("MySettings"));
}

2️⃣ Configure(IApplicationBuilder app, IWebHostEnvironment env)

Purpose: To define the HTTP request pipeline — i.e., the sequence of middleware that processes incoming requests and outgoing responses.

What happens here:

• You add middleware components that handle requests (e.g., authentication, routing, static files).
• You decide the order in which middleware runs.
• You can branch the pipeline based on environment (Development, Production).

Key points:

• Runs after ConfigureServices.
• Uses app.Use..., app.Run..., and app.Map... methods.
• Middleware order is critical — it affects how requests are processed.

Example:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthentication();
    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}

🔍 How They Work Together

Step	Method	Purpose
1	ConfigureServices	Register all services and dependencies your app needs.
2	Configure	Build the middleware pipeline that uses those services to handle requests.

Think of it like this:

• ConfigureServices = "What tools and helpers will my app have?"
• Configure = "In what order will my app use those tools to process a request?"

💡 Pro Tip

In .NET 6+ minimal hosting model, these two methods are often replaced by calls on the builder.Services and app objects in Program.cs, but the concepts are exactly the same — service registration first, pipeline configuration second.