Program.cs

In simple words#

Learning objectives#

• Read the four phases of Program.cs
• Register services on builder.Services
• Configure the pipeline differently per environment
• Map MVC routes and static assets

Explanation#

WebApplication.CreateBuilder(args) creates a builder with sensible defaults: Kestrel, configuration from appsettings.json, environment variables and command-line args, logging to the console, and a dependency injection container.

Phase 1 — services: builder.Services.AddControllersWithViews(), AddDbContext, AddScoped<IMyService, MyService>()… Everything your app needs is registered here, before Build().

Phase 2 — build: var app = builder.Build(); After this, the service list is locked.

Phase 3 — pipeline: app.UseExceptionHandler, UseHsts, UseHttpsRedirection, UseRouting, UseAuthentication, UseAuthorization… Order matters; production-only middleware sits inside if (!app.Environment.IsDevelopment()).

Phase 4 — endpoints and run: MapStaticAssets() serves wwwroot with fingerprinting and caching (.NET 9), MapControllerRoute defines the conventional route, and app.Run() starts listening until the app is stopped.

Syntax#

1var builder = WebApplication.CreateBuilder(args);   // defaults: Kestrel, config, logging, DI
2builder.Services.Add…();                             // 1. services
3var app = builder.Build();                           // 2. build
4app.Use…();                                          // 3. middleware pipeline
5app.Map…();                                          // 4. endpoints
6app.Run();

Code example: ShopSmart’s Program.cs#

1// Program.cs
2var builder = WebApplication.CreateBuilder(args);
3

4// ── 1. Register services ──
5builder.Services.AddControllersWithViews();
6builder.Services.AddScoped<IProductCatalog, InMemoryProductCatalog>();
7

8// ── 2. Build the app ──
9var app = builder.Build();
10

11// ── 3. Configure the HTTP request pipeline ──
12if (!app.Environment.IsDevelopment())
13{
14    app.UseExceptionHandler("/Home/Error");   // friendly error page in production
15    app.UseHsts();                            // tell browsers to always use HTTPS
16}
17app.UseHttpsRedirection();
18app.UseRouting();
19app.UseAuthorization();
20

21// ── 4. Map endpoints and run ──
22app.MapStaticAssets();                        // wwwroot files, fingerprinted and cached
23app.MapControllerRoute(
24        name: "default",
25        pattern: "{controller=Home}/{action=Index}/{id?}")
26    .WithStaticAssets();
27

28app.Run();
29

30// Services/IProductCatalog.cs
31public interface IProductCatalog
32{
33    IReadOnlyList<string> GetNames();
34}
35

36public class InMemoryProductCatalog : IProductCatalog
37{
38    public IReadOnlyList<string> GetNames() => ["Wireless Mouse", "USB-C Hub", "Mechanical Keyboard"];
39}

The app starts, prints “Now listening on: https://localhost:7046”, and serves ShopSmart. In Development, errors show the detailed developer exception page; in Production they go to /Home/Error.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. CreateBuilder sets up configuration, logging, Kestrel and DI.
2. Services are registered on builder.Services before Build(); IProductCatalog will be injected wherever it is requested.
3. The pipeline is configured on app: production-only middleware first, then HTTPS redirection, routing and authorization.
4. MapStaticAssets and the default controller route connect URLs to files and controllers.
5. app.Run() blocks and serves requests until the process stops.

Real-world use case#

Common mistakes#

Best practices#

• Keep the four phases visually separate
• Wrap production-only middleware in an environment check
• Move groups of registrations into extension methods
• Use MapStaticAssets in .NET 9+ for cache-friendly static files

Remember this#

Practice exercise#

1// Program.cs
2var builder = WebApplication.CreateBuilder(args);
3builder.Services.AddControllersWithViews();
4builder.Services.AddShopServices();
5var app = builder.Build();
6app.MapDefaultControllerRoute();
7app.Run();
8

9// Extensions/ServiceCollectionExtensions.cs
10public static class ServiceCollectionExtensions
11{
12    public static IServiceCollection AddShopServices(this IServiceCollection services)
13    {
14        services.AddScoped<IProductCatalog, InMemoryProductCatalog>();
15        return services;
16    }
17}
18

19public interface IProductCatalog { IReadOnlyList<string> GetNames(); }
20public class InMemoryProductCatalog : IProductCatalog
21{
22    public IReadOnlyList<string> GetNames() => ["Wireless Mouse"];
23}

Mini quiz#

Want more? Practise all ASP.NET Core MVC MCQs · ASP.NET Core MVC interview questions

Related topics#

My notes#

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