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How do complex/composite route segments (combining literal text and a parameter in the same segment, like {fileName}.{extension}) work? How would you implement a custom route constraint (IRouteConstraint) for a domain-specific validation pattern, like a product SKU format? How do you write custom middleware or an endpoint filter that reads endpoint metadata to conditionally alter its behavior per-route? How would you implement a catch-all route parameter to handle a variable-depth URL path, like a file browser or wiki-style page hierarchy? How do you generate a URL for a named route or endpoint programmatically, rather than hardcoding URL strings throughout the application (like in a Location header for a 201 Created response)? How would you debug why a specific request isn't matching the route you expect it to, using ASP.NET Core's routing diagnostics? How would you design a route structure and constraint strategy for a large, RESTful API with deeply nested resource hierarchies (like organizations/{orgId}/projects/{projectId}/tasks/{taskId})?
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ASP.NET Core › Endpoint Metadata, Route Constraints & Templates

How would you implement route-based API versioning using route templates and constraints without a dedicated versioning library?

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Answer

A hand-rolled approach embeds the version directly in the route template ({version} segment, potentially constrained to a specific set of allowed values via a custom constraint or regex constraint) and dispatches to different handler logic based on the captured version value -- while workable for simple cases, this reimplements much of what a dedicated library like Asp.Versioning already provides more robustly (deprecation headers, Swagger integration), so it's generally only justified for very simple versioning needs.
app.MapGet("/api/v{version:regex(^(1|2)$)}/products/{id}", (string version, int id) => {
    return version == "2" ? GetProductV2(id) : GetProductV1(id);
});
Real-world example A small internal API implements minimal hand-rolled route-based versioning using a regex constraint restricting the version segment to '1' or '2', appropriate for its narrow, simple versioning needs without pulling in a full versioning library's overhead.

Common follow-ups: At what point does hand-rolled versioning become worth replacing with a dedicated library like Asp.Versioning?;How does this approach handle version-specific Swagger documentation?

API Versioning;API Documentation with Swagger/OpenAPI
All 15 Endpoint Metadata, Route Constraints & Templates questions