# This week in Cursor + .NET — 3 rules + 4 essays (week ending May 22, 2026) > Source: > Published: 2026-05-22 21:32:35+00:00 Every weekday a single, opinionated rule for senior C#/.NET engineers using Cursor. Here's the full week in one read — canonical posts live on the Agentic Architect blog. Thu 21 May · C# patterns Stop the AI from undoing your Result/ErrorOr error model with throw and null on every new prompt. Wed 20 May · Cursor rules The correction spiral — and the circuit-breaker rule that ends it before you type the stop phrase. Tue 19 May · .NET DI The .NET lifetime bug that ships to production silently — and the one Cursor rule that catches it before merge. Mon 18 May · Architecture The hidden cost senior engineers pay every morning — and a four-rule architecture for eliminating it. Fri 22 May The single most expensive .NET runtime bug: a Singleton holding a Scoped service. Cursor cheerfully writes this without warning. Audit constructor parameters of any class registered as Singleton — if any are typically Scoped (DbContext, repositories, MediatR sender), flag it before merge. Thu 21 May Every async method in your codebase should accept and forward a CancellationToken. Make it a rule that any new async signature without one is flagged. Stops the AI from quietly losing cancellation half-way down a call chain. Wed 20 May Never let the AI write database logic in your Controllers. Enforce a strict boundary where all IQueryable access stays in the Infrastructure layer. This keeps your business logic "pure" and readable for the LLM. The free arch-core-lite.mdc is one drop-in Cursor rule that ends the morning re-explanation ritual. Install in 60 seconds, see whether Cursor actually remembers your DI lifetimes, and decide for yourself whether the full kit is worth £19.99. arch-core-lite.mdc on GitHub Canonical home for everything in this digest: https://agenticstandardcontact-byte.github.io/agentic-architect/blog/.