Legacy .NET, moved forward without stopping the business

The page lifecycle, ViewState and server controls have no equivalent, so the markup genuinely is rewritten. The business logic underneath it usually is not — and pulling the two apart is most of the job.

Bindings, behaviours and SOAP contracts do not port. Where the callers are third parties you cannot change, CoreWCF keeps the contract alive while everything behind it moves.

There is nothing to migrate to — the runtime is gone from every browser. These start as a scoping conversation about what the application actually needed to do.

The data layer is where a straightforward port turns expensive. Query translation, transactions and lazy loading all behave differently, and it shows up in production rather than at compile time.

The closest thing to a mechanical port on this list. The real work is hosting, configuration, dependency injection and the middleware pipeline — not your controllers.

Both still run on modern .NET, so this is rarely a rewrite. What blocks it is abandoned third-party control suites and COM interop, which is the first thing we go looking for.

Shared libraries are what let the old and new systems run side by side. Getting them multi-targeting first is usually the unlock for everything else on this list.

Unglamorous, and it is what makes the rest repeatable. It also pays off long before the migration finishes, which is why it is often the right thing to do first.