Why should you care?

It is cross-platform (Windows, Linux, Mac), fast, and the standard choice for new .NET backends in 2025–2026. Companies migrating from .NET Framework Web API 2 use ASP.NET Core.

See it live — copy this example

Create a Web API (dotnet new webapi), paste the example, run dotnet run, test in Swagger.

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();
app.UseSwagger();
app.UseSwaggerUI();
app.MapControllers();
app.Run();

Run Example »

This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.

What happened?

• CreateBuilder sets up dependency injection.
• AddControllers registers API controllers.
• MapControllers connects routes.
• Swagger gives you a test UI in development.

Try it yourself

1. Create ShopNest.API with dotnet new webapi.
2. Open Program.cs and read each line — match it to the diagram in this lesson.
3. Add a ProductsController with one GET action.
4. Change a route URL or DTO property and save — test again in Swagger or curl.
5. Return the wrong status code on purpose (404 instead of 200) and see what the client shows.

Remember

Program.cs = DI + middleware pipeline + Run(). Controllers live in Controllers/ folder with [ApiController]. Swagger helps you test without writing a frontend first.