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<T>/Memory<T> 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

Language Patterns

Records for Immutable Data (C# 9+)

Use

record

// 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

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

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 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 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 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 async IAsyncEnumerable<Order> StreamOrdersAsync(
    string customerId,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    await foreach (var order in _repository.StreamAllAsync(cancellationToken))
    {
        if (order.CustomerId == customerId)
            yield return order;
    }
}

Key rules:

• Always accept

  CancellationToken

  with

  = default

• Use

  ConfigureAwait(false)

  in library code
• Never block on async code (no

  .Result

  or

  .Wait()

  )
• Use linked CancellationTokenSource for timeouts

Span<T> and Memory<T>

Use

Span<T>

Memory<T>

ArrayPool<T>

See performance-and-api-design.md for complete Span/Memory examples and the API design section.

Error Handling: Result Type

For expected errors, use

Result<T, TError>

See composition-and-error-handling.md for the full Result type implementation and usage examples.

Avoid Reflection-Based Metaprogramming

Banned: AutoMapper, Mapster, ExpressMapper. Use explicit mapping extension methods instead. Use

UnsafeAccessorAttribute

See anti-patterns-and-reflection.md for full guidance.

Code Organization

// File: Domain/Orders/Order.cs

namespace MyApp.Domain.Orders;

// 1. Primary domain type
public record Order(
    OrderId Id,
    CustomerId CustomerId,
    Money Total,
    OrderStatus Status,
    IReadOnlyList<OrderItem> Items
)
{
    public bool IsCompleted => Status is OrderStatus.Completed;

    public Result<Order, OrderError> AddItem(OrderItem item)
    {
        if (Status is not OrderStatus.Draft)
            return Result<Order, OrderError>.Failure(
                new OrderError("ORDER_NOT_DRAFT", "Can only add items to draft orders"));

        var newItems = Items.Append(item).ToList();
        var newTotal = new Money(
            Items.Sum(i => i.Total.Amount) + item.Total.Amount,
            Total.Currency);

        return Result<Order, OrderError>.Success(
            this with { Items = newItems, Total = newTotal });
    }
}

// 2. Enums for state
public enum OrderStatus { Draft, Submitted, Processing, Completed, Cancelled }

// 3. Related types
public record OrderItem(ProductId ProductId, Quantity Quantity, Money UnitPrice)
{
    public Money Total => new(UnitPrice.Amount * Quantity.Value, UnitPrice.Currency);
}

// 4. Value objects
public readonly record struct OrderId(Guid Value)
{
    public static OrderId New() => new(Guid.NewGuid());
}

// 5. Errors
public readonly record struct OrderError(string Code, string Message);

Best Practices Summary

DO's

• Use

  record

  for DTOs, messages, and domain entities
• Use

  readonly record struct

  for value objects
• Leverage pattern matching with

  switch

  expressions
• Enable and respect nullable reference types
• Use async/await for all I/O operations
• Accept

  CancellationToken

  in all async methods
• Use

  Span<T>

  and

  Memory<T>

  for high-performance scenarios
• Accept abstractions (

  IEnumerable<T>

  ,

  IReadOnlyList<T>

  )
• Use

  Result<T, TError>

  for expected errors
• Pool buffers with

  ArrayPool<T>

  for large allocations
• Prefer composition over inheritance

DON'Ts

• Don't use mutable classes when records work
• Don't use classes for value objects (use

  readonly record struct

  )
• Don't create deep inheritance hierarchies
• Don't ignore nullable reference type warnings
• Don't block on async code (

  .Result

  ,

  .Wait()

  )
• Don't use

  byte[]

  when

  Span<byte>

  suffices
• Don't forget

  CancellationToken

  parameters
• Don't return mutable collections from APIs
• Don't throw exceptions for expected business errors
• Don't allocate large arrays repeatedly (use

  ArrayPool

  )

See anti-patterns-and-reflection.md for detailed anti-pattern examples.

Additional Resources

• C# Language Specification: https://learn.microsoft.com/en-us/dotnet/csharp/
• Pattern Matching: https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching
• Span<T> and Memory<T>: https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/
• Async Best Practices: https://learn.microsoft.com/en-us/archive/msdn-magazine/2013/march/async-await-best-practices-in-asynchronous-programming
• .NET Performance Tips: https://learn.microsoft.com/en-us/dotnet/framework/performance/