This week in Cursor + .NET — 3 rules + 4 essays (week ending May 22, 2026)

#csharp #cursor #ai #dotnet

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.

New essays this week

Teach Cursor Result<T> instead of throwing

Thu 21 May · C# patterns

Stop the AI from undoing your Result/ErrorOr error model with throw and null on every new prompt.

Seven words that stop Cursor hallucination loops

Wed 20 May · Cursor rules

The correction spiral — and the circuit-breaker rule that ends it before you type the stop phrase.

The Scoped→Singleton DI bug your AI just suggested (and how to catch it)

Tue 19 May · .NET DI

The .NET lifetime bug that ships to production silently — and the one Cursor rule that catches it before merge.

The Context Tax: Why every Cursor session costs you 15 minutes

Mon 18 May · Architecture

The hidden cost senior engineers pay every morning — and a four-rule architecture for eliminating it.

3 daily senior rules

Rule 9: Scoped Capture in Singleton

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.

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Rule 8: CancellationToken Propagation

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.

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Rule 2: Persistence Boundaries

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.

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Try one rule before you trust the whole kit

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.