A route parameter, also called a segment variable, is a named placeholder in a URL pattern. Unlike a literal segment such as `capital`, `{country}` can match different path segments. ASP.NET Core routing passes the captured value to the selected endpoint through `HttpRequest.RouteValues`. Names such as `action`, `area`, `controller`, `handler`, and `page` are reserved. section 13.1.5
Read a value in an endpoint
Register `app.MapGet("capital/{country}", Capital.Endpoint);`. A request for `/capital/uk` supplies `uk` under the key `country`; the endpoint can read it with `context.Request.RouteValues["country"] as string`. A matching `population/{city}` route can supply a different value to another endpoint. Restart ASP.NET Core and request a matching URL to observe the response. section 13.1.5
Choose a pattern that captures exactly what the endpoint needs:
- `{first}/{second}/{third}` captures three path segments. Enumerate `RouteValues` to inspect their names and values; `/apples/oranges/cherries` provides a quick test. section 13.1.5
- `files/{filename}.{ext}` captures two values from one segment. The literal period separates them, so `/files/myfile.txt` supplies a filename and extension. section 13.2.1
- Keep complex patterns testable. A pattern beginning `example/red{color}` matches `/example/redgreen`, but the described matching behavior fails for `/example/redredgreen`; such failures can change between ASP.NET Core releases. section 13.2.1
The same pattern feature reaches other ASP.NET Core entry points. A Razor Page can use `@page "{id:long?}"` to make an `id` segment optional and constrain it to a value parseable as `long`; its handler can receive the value through model binding. A Razor Component can use `@page "/person/{id:long}"` and a matching `[Parameter] public long Id { get; set; }` property. Without a type in the component’s `@page` expression, the property must be a string. section 23.3.1 section 35.2.4
Route values can also drive web-service behavior. A minimal API route under `/api/products/{id}` uses the captured ID to look up one product. In a controller route, an optional `{format?}` value works with `[FormatFilter]` to select JSON or XML from the URL instead of the request’s `Accept` header. section 19.3 section 20.4.4
Listing 13.11 Updating routes in the Program.cs file in the Platform folder
using Platform;
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
app.MapGet("{first}/{second}/{third}", async context => {
await context.Response.WriteAsync("Request Was Routed\n");
foreach (var kvp in context.Request.RouteValues) {
await context.Response
.WriteAsync($"{kvp.Key}: {kvp.Value}\n");
}
});
app.MapGet("capital/{country}", Capital.Endpoint);
app.MapGet("population/{city}", Population.Endpoint);
app.Run();
The complete `Program.cs` listing shows parameterized route registrations beside an endpoint that prints captured route values. section 13.1.5
