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Plenty of business-critical software still runs on .NET Framework 4.x. It works, but it ties you to Windows hosting, it no longer gets new features and it is getting harder to hire for. The good news: moving to .NET 10 does not require a big-bang rewrite. ## Why migrate now - Hosting: .NET 10 runs on Linux and in containers, which is usually cheaper and easier to scale. - Performance: modern .NET is significantly faster for typical web and API workloads. - Security and support: .NET 10 is a long-term support release with regular patches. - Hiring: developers want to work on current versions of the platform. ## Step 1: inventory the solution List every project, its target framework and its NuGet packages. Flag anything that depends on System.Web, WCF server, Web Forms or Windows-only APIs. These are your hard parts, and knowing them up front keeps the estimate honest. ## Step 2: move the libraries first Class libraries with business logic usually have few platform dependencies. Retarget them to netstandard2.0 or multi-target net48 and net10.0 so both the old app and the new one can use them while you work. ## Step 3: put tests around the risky parts Before changing the web layer, add integration tests around the flows that make money: checkout, invoicing, reporting. They turn a scary migration into a series of small, verifiable steps. ## Step 4: migrate the web layer route by route Use the strangler-fig pattern: run a new ASP.NET Core app alongside the old one and move routes across in small groups, with a reverse proxy (such as YARP) sending traffic to whichever app owns each route. Users never see a cut-over weekend. - Start with simple, read-only pages to prove the setup. - Move authentication early so both apps share the same sign-in. - Retire the old app once the last route has moved. ## Step 5: modernise as you go, not before Resist rewriting everything "properly" during the move. Swap Entity Framework 6 for EF Core, add dependency injection and introduce async where it helps, but only in the code you are already touching. ## What it usually takes A mid-sized app with a handful of projects typically moves in a few months of part-time work alongside normal feature development. Tools such as the .NET Upgrade Assistant speed up the mechanical parts; the real time goes into the System.Web dependencies you found in step 1. Planning a migration? We are happy to review your solution and give you a step-by-step plan and estimate.

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