FakeTimeProvider gives you a clock you control. That solves the easy 30%.
The hard part is everything that interacts with that clock: async callbacks,
PeriodicTimer, CancellationTokenSource.CancelAfter, BackgroundService loops,
and DI lifetime traps that turn deterministic bugs into apparently flaky tests.
Directory.Build.props drift is the quiet tax every multi-repo .NET org pays. NetEvolve.Defaults ships MSBuild properties, .editorconfig, NuGet Audit, and ten Roslyn diagnostics as a private-asset NuGet package. Bump the version once, every repo gets the upgrade.
STS releases are supposed to be quiet bridges to the next LTS. .NET 11 is not. The union keyword landed in Preview 2, runtime-async is now the default for the BCL and ASP.NET Core shared framework, System.Diagnostics.Process ships a major overhaul, and EF Core gains approximate vector search against SQL Server 2025. All facts sourced from the .NET repositories.
Most .NET test projects rely on NSubstitute and Castle.DynamicProxy. That foundation is cracking: NativeAOT breaks it, trimming strips it, cold-start costs accumulate. TUnit.Mocks takes a different route: a source generator emits typed mocks at compile time, IService.Mock() (or Mock.Of<T>()) is the entry point, runtime reflection is gone.
The first four parts of this series treated legacy as something between me and myself: Past Self leaving code for Future Self, with an AI in the middle. That framing is incomplete. Code outlives employment, not just memory. The companies I worked for have forgotten most of what I did there. The repositories haven’t. This is the fifth part of the Code as Legacy series, about the legacy you leave when you’re not around to defend it.
