modern-csharp-coding-standards

Modern C# Coding Standards

When to Use This Skill

Use this skill when:

• Writing new C# code or refactoring existing code
• Designing public APIs for libraries or services
• Optimizing performance-critical code paths
• Implementing domain models with strong typing
• Building async/await-heavy applications
• Working with binary data, buffers, or high-throughput scenarios

Reference Files

• value-objects-and-patterns.md: Full value object examples and pattern matching code
• performance-and-api-design.md: Span/Memory examples and API design principles
• composition-and-error-handling.md: Composition over inheritance, Result type, testing patterns
• anti-patterns-and-reflection.md: Reflection avoidance and common anti-patterns

Core Principles

1. Immutability by Default - Use record types and init-only properties
2. Type Safety - Leverage nullable reference types and value objects
3. Modern Pattern Matching - Use switch expressions and patterns extensively
4. Async Everywhere - Prefer async APIs with proper cancellation support
5. Zero-Allocation Patterns - Use Span<T> and Memory<T> for performance-critical code
6. API Design - Accept abstractions, return appropriately specific types
7. Composition Over Inheritance - Avoid abstract base classes, prefer composition
8. Value Objects as Structs - Use readonly record struct for value objects

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Language Patterns

Records for Immutable Data (C# 9+)

Use record types for DTOs, messages, events, and domain entities.

// Simple immutable DTO
public record CustomerDto(string Id, string Name, string Email);

// Record with validation in constructor
public record EmailAddress
{
    public string Value { get; init; }

    public EmailAddress(string value)
    {
        if (string.IsNullOrWhiteSpace(value) || !value.Contains('@'))
            throw new ArgumentException("Invalid email address", nameof(value));

        Value = value;
    }
}

// Records with collections - use IReadOnlyList
public record ShoppingCart(
    string CartId,
    string CustomerId,
    IReadOnlyList<CartItem> Items
)
{
    public decimal Total => Items.Sum(item => item.Price * item.Quantity);
}