Migrating from .NET Framework to .NET 10: A Step-by-Step Modernisation Guide
How do you move a legacy .NET Framework app to .NET 10? Why migrate, which strategy to pick and how to manage risk. A practical guide.
Plenty of businesses still run critical applications built on .NET Framework 4.x. They've done their job for years, but they no longer get new features, run only on Windows and struggle to fit modern cloud infrastructure. This guide walks you through planning a low-risk migration from .NET Framework to .NET 10.
Why migrate now?
- Performance: every modern .NET release ships major performance gains. You can serve far more requests on the same hardware.
- Long-term support: .NET 10 is an LTS release and receives security updates for years.
- Cross-platform: your app can run on Linux servers and in Docker containers, cutting licence and hosting costs.
- Talent: developers want to work with modern .NET; staffing legacy stacks gets harder every year.
- Security: current authentication standards, data-protection APIs and secure defaults.
Step 1: Inventory and analysis
Before migrating, take a snapshot of what you have:
- Which project types exist? (Web Forms, MVC, WCF, Windows Services, WinForms...)
- Which NuGet packages and third-party libraries are used, and do modern versions exist?
- Where are the Windows-specific dependencies (Registry, COM, System.Web)?
- What does test coverage look like?
Microsoft's .NET Upgrade Assistant is a good starting point, but uncovering the real risks requires an experienced review of the code.
Step 2: Choose the right strategy
In-place upgrade
Suitable for small and mid-sized, well-layered applications. Project files are converted to SDK style, the target framework is updated and incompatible APIs are fixed.
Incremental migration (strangler fig)
The safest approach for large, critical systems. A new ASP.NET Core app sits in front of the legacy system behind a reverse proxy such as YARP. Modules move across one by one, while requests that haven't been migrated still reach the old app. The business never stops.
Rewrite
When source code is lost, the architecture is unmaintainable or requirements have fundamentally changed, extracting the business rules and rebuilding on a modern architecture can be more economical.
Step 3: Move shared code first
Class libraries containing business logic are usually first to move. They can be multi-targeted so both the old and new applications can use them during the transition.
<TargetFrameworks>net48;net10.0</TargetFrameworks>Step 4: Move the web layer to ASP.NET Core
System.Web doesn't exist in modern .NET, so:
- MVC controllers move to ASP.NET Core MVC with a largely similar structure.
- Razor Pages or Blazor are good replacements for Web Forms.
- For WCF services, consider CoreWCF, gRPC or REST APIs.
web.configsettings move toappsettings.jsonand environment variables.
Step 5: Data access
If you use Entity Framework 6, you can first keep EF6 running on modern .NET and later move to EF Core. For stored-procedure-heavy systems, Dapper is a fast and practical intermediate option.
Step 6: Test, observe and release
The most critical part of a migration is proving behaviour hasn't changed. Write integration tests for key workflows, release the new system to limited traffic first (canary) and monitor performance and error rates with tools like OpenTelemetry.
Common mistakes
- Treating the migration as a side project without a firm timeline.
- Trying to move everything at once ("big bang").
- Refactoring without tests.
- Rewriting all business rules at the same time as migrating.
Conclusion
Planned properly, migrating from .NET Framework to .NET 10 reduces technical debt and gives your business speed, security and cost advantages. Curious how ready your application is? Explore our legacy modernisation service or get in touch for a free initial assessment.