/csharp-coding-standards

name: modern-csharp-coding-standards
description: Write modern, high-performance C# code using records, pattern matching, value objects, async/await, Span<T>/Memory<T>, and best-practice API design patterns. Emphasizes functional-style programming with C# 12+ features.
invocable: false

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](value-objects-and-patterns.md): Full value object examples and pattern matching code
• [performance-and-api-design.md](performance-and-api-design.md): Span<T>/Memory<T> examples and API design principles
• [composition-and-error-handling.md](composition-and-error-handling.md): Composition over inheritance, Result type, testing patterns
• [anti-patterns-and-reflection.md](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

---

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);
}

**When to use `record class` vs `record struct`:**

• `record class` (default): Reference types, use for entities, aggregates, DTOs with multiple properties
• `record struct`: Value types, use for value objects (see next section)

Value Objects as readonly record struct

Value objects should **always be `readonly record struct`** for performance and value semantics. Use explicit conversions, never implicit operators.

public readonly record struct OrderId(string Value)
{
    public OrderId(string value) : this(
        !string.IsNullOrWhiteSpace(value)
            ? value
            : throw new ArgumentException("OrderId cannot be empty", nameof(value)))
    { }
    public override string ToString() => Value;
}

public readonly record struct Money(decimal Amount, string Currency);
public readonly record struct CustomerId(Guid Value)
{
    public static CustomerId New() => new(Guid.NewGuid());
}

See [value-objects-and-patterns.md](value-objects-and-patterns.md) for complete examples including multi-value objects, factory patterns, and the no-implicit-conversion rule.

Pattern Matching (C# 8-12)

Use switch expressions, property patterns, relational patterns, and list patterns for cleaner code.

public decimal CalculateDiscount(Order order) => order switch
{
    { Total: > 1000m } => order.Total * 0.15m,
    { Total: > 500m } => order.Total * 0.10m,
    { Total: > 100m } => order.Total * 0.05m,
    _ => 0m
};

See [value-objects-and-patterns.md](value-objects-and-patterns.md) for full pattern matching examples.

---

Nullable Reference Types (C# 8+)

Enable nullable reference types in your project and handle nulls explicitly.

// In .csproj
<PropertyGroup>
    <Nullable>enable</Nullable>
</PropertyGroup>

// Explicit nullability
public string? FindUserName(string userId)
{
    var user = _repository.Find(userId);
    return user?.Name;
}

// Pattern matching with null checks
public decimal GetDiscount(Customer? customer) => customer switch
{
    null => 0m,
    { IsVip: true } => 0.20m,
    { OrderCount: > 10 } => 0.10m,
    _ => 0.05m
};

// Guard clauses with ArgumentNullException.ThrowIfNull (C# 11+)
public void ProcessOrder(Order? order)
{
    ArgumentNullException.ThrowIfNull(order);
    // order is now non-nullable in this scope
    Console.WriteLine(order.Id);
}

---

Composition Over Inheritance

**Avoid abstract base classes.** Use interfaces + composition. Use static helpers for shared logic. Use records with factory methods for variants.

See [composition-and-error-handling.md](composition-and-error-handling.md) for full examples.

---

Performance Patterns

Async/Await Best Practices

// Async all the way - always accept CancellationToken
public async Task<Order> GetOrderAsync(string orderId, CancellationToken cancellationToken)
{
    var order = await _repository.GetAsync(orderId, cancellationToken);
    return order;
}

// ValueTask for frequently-called, often-synchronous methods
public ValueTask<Order?> GetCachedOrderAsync(string orderId, CancellationToken cancellationToken)
{
    if (_cache.TryGetValue(orderId, out var order))
        return ValueTask.FromResult<Order?>(order);
    return GetFromDatabaseAsync(orderId, cancellationToken);
}

// IAsyncEnumerable for streaming
public a