just-works: Skill for Claude Code

C# Coding

Project conventions override these defaults. Read 2-3 nearby files first, and check the project's own target in the .csproj: TargetFramework, LangVersion, nullable setting, analyzer config. Use only language and library features that target supports (C# 13 needs .NET 9, C# 14 needs .NET 10).

Never rules

• Never write an empty catch, or a catch (Exception) that neither rethrows nor logs. Catch specific types; at a boundary, log and rethrow or convert to a meaningful error.
• Never throw ex. It resets the stack trace. Use throw;, or wrap with an inner exception: throw new XException("msg", ex).
• Never use DateTime.Now for stored or compared timestamps. It is server-local time and breaks across time zones and DST. Take time from an injected TimeProvider (.NET 8+) so tests can control it; otherwise use DateTimeOffset.UtcNow or DateTime.UtcNow. Local time is for display only.
• Never async void except event handlers. It cannot be awaited and its exceptions crash the process.
• Never block on a task with .Result, .Wait(), or .GetAwaiter().GetResult(). It deadlocks in UI and ASP.NET Core contexts. Use await. If a synchronous caller truly cannot be made async, Task.Run(() => MethodAsync()).GetAwaiter().GetResult() avoids the deadlock (the work runs on a thread-pool thread with no SynchronizationContext); add a comment saying why.
• Never use Random for tokens, keys, or session IDs. Use RandomNumberGenerator.GetBytes() / GetInt32().
• Never concatenate or interpolate values into SQL. Use parameters (SqlCommand.Parameters or the ORM's parameterization).
• Avoid mutable static fields that hold request or user state; they are shared across threads. Use DI, IOptions<T>, or a thread-safe type (ConcurrentDictionary, Lock-guarded state). Constants, static readonly values, and lazily built immutable caches are fine.

Resource cleanup

Use using / await using declarations for IDisposable and IAsyncDisposable objects you create. HttpClient is the exception: do not create and dispose one per call (it exhausts ports). Create clients through IHttpClientFactory, or share a long-lived instance with PooledConnectionLifetime set. Implement IDisposable only when the class owns unmanaged resources or other disposables.