1 · Structure and rendering · 30 MIN

Minimal APIs and typed route inputs

Endpoint registration connects an HTTP route to application behavior.

A minimal API maps an HTTP method and path to a delegate. Route constraints such as :int prevent non-integer values from matching that route. A successful match still requires a resource lookup and a deliberate missing-resource response. The example exposes only fixed public metadata. Separating the data access behind a service later makes testing easier and prevents route delegates from becoming large mixtures of validation, persistence and presentation.

Inspect the JSON body and status.

Read the example

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
app.MapGet("/courses/{id:int}", (int id) =>
    id == 1 ? Results.Ok(new {id=1,title="C# foundations"}) : Results.NotFound());
app.Run();

GET /courses/1 returns 200 and the course. /courses/2 and /courses/text return 404 through different paths.

Your challenge

Add a public list endpoint and distinguish invalid query parameters from a valid-but-missing resource.

Solution cost: O(1) lookup in this fixed example. time · O(1) response data. space

Common trap

Route constraints select routes; they are not a complete domain validation system.

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
app.MapGet("/courses/{id:int}", (int id) =>
    id == 1 ? Results.Ok(new {id=1,title="C# foundations"}) : Results.NotFound());
app.Run();

Further reading: Official documentation