.NET Day 2025: The best ways to use the latest OpenAPI features in .NET 9!

Transcript

1. .NET OpenAPI Let’s learn about the best ways to use OpenAPI in .NET 9 and 10! .NET Day Switzerland 2025 Sander ten Brinke https://github.com/sander1095/openapi-dotnet9-talk https://stenbrinke.nl New to OpenAPI or its use-cases? Read my blog post!
2. Sander ten Brinke • Senior Software Engineer & Knowledge Officer @ Arcady • Backend, App, Cloud, DevOps, Frontend.. sander1095 s.tenbrinke2@gmail.com stenbrinke.nl Let’s connect! @sandertenbrinke Sander ten Brinke New to OpenAPI or its use-cases? Read my blog post!
3. Agenda A refresher on OpenAPI OpenAPI in .NET 9 The future of OpenAPI in .NET Conclusion Resources Questions Feel free to interrupt and ask questions! Demos New to OpenAPI or its use-cases? Read my blog post!
4. A refresher on OpenAPI
5. What is OpenAPI? A standard for designing, building, and documenting APIs. SwaggerUI OpenAPI Input for
6. OpenAPI Tooling https://openapi.tools API Client generators Mock servers API testing Documentation 350+ tools! New to OpenAPI or its use-cases? Read my blog post!
7. History of Swagger and OpenAPI 2011 • Swagger 1.0 2014 • Swagger 2.0 2015 • Renamed to OpenAPI 2017 • OpenAPI 3.0.0 2021 • OpenAPI 3.1.0 End of 2024? • OpenAPI 4.0 (Moonwalk)
8. OpenAPI In .NET 9
9. Previously in .NET 8
10. OpenAPI with .NET 9 dotnet add package Microsoft.AspNetCore.OpenApi
11. AOT Support! • Faster application startup! • Smaller binaries and containers! • Less memory usage! • Reflection is problematic for AOT! • Controllers do not support AOT!
12. Breaking changes! SwaggerUI is not included by default (1/2) https://github.com/dotnet/aspnetcore/issues/54598
13. Breaking changes! XML Comments are not supported in .NET 9 https://github.com/dotnet/aspnetcore/issues/39927 https://github.com/martincostello/openapi-extensions (2/2)
14. DEMO Setting up and using OpenAPI in .NET 9 https://github.com/sander1095/openapi-dotnet9-talk
15. Customizing OpenAPI documents Set up OpenAPI specific information •Contact information •ExternalDocs •Servers •Etc… Transform data •Document •Operations •Schemas
16. Generating OpenAPI documents at build-time cat bin/Debug/net9.0/{ProjectName}.json • Read generated OpenAPI document dotnet build • Build the ASP.NET Core application • Add required package dotnet add package Microsoft.Extensions.ApiDescription.Server Why: Store in source control, lint in pipeline, run integration tests...
17. DEMO Customizing OpenAPI documents Build-time document generation https://github.com/sander1095/openapi-dotnet9-talk
18. Finishing up
19. The future of OpenAPI in .NET 10? My PR’s! •I’ve merged 5+ pull requests in ASP.NET Core’s OpenAPI module for .NET 10! XML comment support •Currently in preview! OpenAPI v3.1 support •OpenAPI v4 support is unlikely.. Better build- time generation? •The current package wasn’t built for Microsoft’s own library Other features •Attaching transformers to endpoints directly •YAML support •Bug fixes
20. Should you migrate? It depends…
21. Conclusion .NET 9 and 10 contain lots of changes for OpenAPI! AOT support is one of the biggest changes, next to Microsoft shipping their own OpenAPI library, which changes the way you set up OpenAPI in .NET projects. By decorating your API endpoints royally with OpenAPI metadata, you’ll make your API easier to use, so try to do this much as possible! Scan me or !
22. Useful resources • https://openapi.tools • https://openapis.org • https://tinyurl.com/openapi-dotnet9-docs • https://github.com/dotnet/aspnetcore/ • https://stenbrinke.nl/blog/openapi-api-client-generation • Demo: https://github.com/sander1095/openapi-dotnet9-talk Feel free to take a picture ! Scan me or !
23. Sander ten Brinke Thanks for attending! Questions? Feedback? sander1095 s.tenbrinke2@gmail.com stenbrinke.nl Let’s connect! @sandertenbrinke Scan me or !