.NET Framework 4.8 to .NET 9 Step by Step

How to upgrade a legacy .NET Framework application to .NET 9 incrementally using .NET Standard libraries and modern packages.

How we took our 12-year-old .NET Framework asp.net and windows service application and upgraded it to .NET 9 without rewriting it and continuing to ship during the transition: 1. modernizing .NET Framework pre-upgrade using .NET standard compatible Microsoft.Extensions.* packages 2. converting supporting libraries to .NET standard and creating a consolidated .NET Framework-only library 3. Authoring any new projects in .NET 8+ once dependencies are standard 4. Handling some edge cases using dual compilation for .net framework and .net standard 2.1. 5. ASP.NET mvc and webapi admin site conversion (with AI assistance) 6. Final conversion of remaining projects. 🎥 See more from .NET Conf: aka.ms/dotnetconf-playlist 🔗 Links: .NET 10 — The most intelligent and unified .NET yet: aka.ms/dotnet10/ .NET Aspire 13 — Simplified orchestration for cloud-native apps: aka.ms/aspire13/announcement ASP.NET Core 10 — Modern, secure, and full-stack web development: aka.ms/asp/dotnet/2025 .NET MAUI 10 — Build beautiful apps on every platform: aka.ms/dotnet/maui/2025 C# 14 — A more expressive and productive language: aka.ms/csharp14/announcement/2025 F# 10 — Safer, faster, and more functional than ever: aka.ms/fsharp10/2025/announcement Visual Studio 2026 — Faster, smarter, and built for the future: aka.ms/visualstudio2026/announcement 🎙️ Featuring: Michael Christiansen Connect with .NET: Blog: aka.ms/dotnet/blog Twitter: aka.ms/dotnet/twitter TikTok: aka.ms/dotnet/tiktok Mastodon: aka.ms/dotnet/mastodon LinkedIn: aka.ms/dotnet/linkedin Facebook: aka.ms/dotnet/facebook Docs: learn.microsoft.com/dotnet Forums: aka.ms/dotnet/forums 🙋‍♀️Q&A: aka.ms/dotnet-qa 👨‍🎓Microsoft Learn: aka.ms/learndotnet #dotnet #dotnetconf