Welcome back! So far, you've covered the basics of routing in ASP.NET Core, including working with parameters, literal segments, and using optional and default values. In this lesson, you'll delve into applying constraints to route parameters. These constraints help ensure that only specific values pass through your routes, increasing the robustness and reliability of your applications.

In large applications with numerous routes, it's common for routes to become ambiguous. For example, consider a route like /blog/{year=2024}/{slug?}. This route does not prevent ambiguities because both /blog/2023 (a valid year) and /blog/article-title (a slug) can match.

To solve this, you can define constraints for route parameters to ensure that only specific values pass through your routes. For instance, you can define a constraint to ensure year must be an integer greater than or equal to 2000: /blog/{year:int:min(2000)}/{slug?}.

With this constraint, requests like /blog/article-title will no longer be mistaken for a valid year parameter and will not match this route unless article-title is a valid year. This constraint ensures that only valid years and string slugs are processed correctly, thereby eliminating ambiguities.

Below you can see all route constraints available in ASP.NET Core:

It's also possible to combine multiple constraints by separating them with colons, like so: id:int:min(10)

Let's take a look at constraints in action! In the following example, you'll define two endpoints with different constraints on the id parameter.

The first endpoint will handle requests where id is an integer, such as /todo/123.

The second endpoint will handle requests where id is a valid GUID, such as /todo/123e4567-e89b-12d3-a456-426614174000.

In this lesson, you've explored the importance of route constraints in ASP.NET Core to avoid ambiguities and ensure only valid data routes through your application. By applying constraints to route parameters, you can significantly enhance the security, reliability, and overall user experience of your web applications. Next, you'll get hands-on experience with these concepts through some practical exercises. See you there!