All 15 Endpoint Metadata, Route Constraints & Templates questions
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What is a route template in ASP.NET Core, and how do route parameters like {id} work within one? How do route constraints (like {id:int} or {id:guid}) restrict which requests match a given route, and why are they useful? What is endpoint metadata, and how do attributes like [Authorize] and custom metadata attach to a specific route for later inspection by middleware? How does route template precedence work when multiple routes could potentially match the same incoming URL? What is route grouping via MapGroup, and how does it let you apply shared route prefixes, metadata, and filters to a set of related minimal API endpoints? How do route parameter transformers let you customize the casing convention of generated URLs, such as converting PascalCase action names to kebab-case URLs? 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?
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ASP.NET Core › Endpoint Metadata, Route Constraints & Templates

How do you define optional route parameters, and what's the difference between an optional parameter and one with a default value?

Intermediate

Answer

Appending a question mark to a route parameter ({id?}) makes it optional, resulting in a null/default value in the handler if the URL segment is omitted entirely. A default value (using = syntax in older routing conventions, or a C# default parameter value in minimal APIs) instead provides a specific fallback value rather than null when the segment is absent -- both let one route template match URLs with or without that particular segment present.
// Optional: results in null if omitted
app.MapGet("/products/{category?}", (string? category) =>
    category is null ? GetAllProducts() : GetByCategory(category));

// Default value: results in "all" if omitted, via C# default parameter
app.MapGet("/products/{category}", (string category = "all") => GetByCategory(category));
Real-world example A product listing endpoint supports both /products (showing everything) and /products/electronics (filtered) using a single optional route parameter, avoiding the need for two separately registered route templates.

Common follow-ups: How does an optional route parameter interact with route constraints applied to it?;What happens if an optional parameter appears before a required one in the template?

Model Binding & Validation;RESTful Web APIs & Controllers
All 15 Endpoint Metadata, Route Constraints & Templates questions