dotnet explains how their team successfully migrated a 12-year-old .NET Framework application to .NET 9, outlining each critical step and the technical considerations involved.

.NET Framework 4.8 to .NET 9 Step by Step Migration Guide

This session presents a comprehensive strategy for upgrading an aging .NET Framework ASP.NET and Windows Service application (over 12 years in production) directly to .NET 9 without a complete rewrite. The process maintains the ability to ship software throughout the transition, balancing risk and modernization goals.

Key Migration Steps

Pre-Upgrade Modernization
- Use .NET Standard-compatible Microsoft.Extensions.* packages in the .NET Framework application to enable gradual migration.
Library Conversion
- Convert existing supporting libraries to .NET Standard where feasible.
- For legacy dependencies, create a consolidated library that remains .NET Framework-specific.
Greenfield Projects in .NET 8+
- Any new projects or major new features are authored in .NET 8 or later, provided dependencies support .NET Standard.
Dual Compilation Techniques
- Employ dual compilation for components that must support both .NET Framework and .NET Standard 2.1, handling edge cases effectively.
Modernizing ASP.NET Admin Site
- The admin site (based on MVC/WebAPI) is converted—a process that benefited from AI coding assistance tools for faster, more reliable migration.
Final Project Conversion
- Remaining projects are iteratively upgraded, with careful planning to mitigate risk and avoid downtime.

Lessons Learned

Continuous shipping during modernization reduces business risk.
Incremental upgrades using .NET Standard can ease migration of large, complex codebases.
AI-assisted refactoring (for example, during the admin site update) can accelerate transition while maintaining quality.

Additional Resources

Session video: Watch on YouTube
Explore more .NET Conf 2025 sessions for further migration and modernization insights.