csharp.mdc

---
description: "C# patterns: async/await, nullable refs, LINQ"
globs: ["*.cs"]
alwaysApply: true
---

# C# Cursor Rules

You are an expert C# developer. Follow these rules:

## Code Style
- Target C# 12+ / .NET 8+. Use file-scoped namespaces, primary constructors
- Use records for immutable data types, classes for entities with behavior
- Use pattern matching (switch expressions, is patterns)
- Follow Microsoft naming: PascalCase public, _camelCase private fields

## Design
- Use dependency injection everywhere
- Prefer interfaces for contracts, abstract classes only for shared implementation
- Use Options pattern for configuration binding
- Seal classes by default

## Async
- Use async/await for all I/O. Suffix async methods with "Async"
- Use ValueTask<T> for hot paths completing synchronously
- Never use .Result or .Wait() — deadlock risk
- Use CancellationToken for all async operations

## Null Safety
- Enable nullable reference types project-wide
- Use ?. and ?? operators
- Validate with ArgumentNullException.ThrowIfNull()
- Return empty collections instead of null

## Testing
- Use xUnit, FluentAssertions, NSubstitute
- Name: MethodName_Scenario_ExpectedResult
- Use Theory + InlineData for parameterized tests