csharp-type-design-performance

Type Design for Performance

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Install skill "csharp-type-design-performance" with this command: npx skills add foxminchan/bookworm/foxminchan-bookworm-csharp-type-design-performance

Type Design for Performance

When to Use This Skill

Use this skill when:

  • Designing new types and APIs

  • Reviewing code for performance issues

  • Choosing between class, struct, and record

  • Working with collections and enumerables

Core Principles

  • Seal your types - Unless explicitly designed for inheritance

  • Prefer readonly structs - For small, immutable value types

  • Prefer static pure functions - Better performance and testability

  • Defer enumeration - Don't materialize until you need to

  • Return immutable collections - From API boundaries

Seal Classes by Default

Sealing classes enables JIT devirtualization and communicates API intent.

// DO: Seal classes not designed for inheritance public sealed class OrderProcessor { public void Process(Order order) { } }

// DO: Seal records (they're classes) public sealed record OrderCreated(OrderId Id, CustomerId CustomerId);

// DON'T: Leave unsealed without reason public class OrderProcessor // Can be subclassed - intentional? { public virtual void Process(Order order) { } // Virtual = slower }

Benefits:

  • JIT can devirtualize method calls

  • Communicates "this is not an extension point"

  • Prevents accidental breaking changes

Readonly Structs for Value Types

Structs should be readonly when immutable. This prevents defensive copies.

// DO: Readonly struct for immutable value types public readonly record struct OrderId(Guid Value) { public static OrderId New() => new(Guid.NewGuid()); public override string ToString() => Value.ToString(); }

// DO: Readonly struct for small, short-lived data public readonly struct Money { public decimal Amount { get; } public string Currency { get; }

public Money(decimal amount, string currency)
{
    Amount = amount;
    Currency = currency;
}

}

// DON'T: Mutable struct (causes defensive copies) public struct Point // Not readonly! { public int X { get; set; } // Mutable! public int Y { get; set; } }

When to Use Structs

Use Struct When Use Class When

Small (≤16 bytes typically) Larger objects

Short-lived Long-lived

Frequently allocated Shared references needed

Value semantics required Identity semantics required

Immutable Mutable state

Prefer Static Pure Functions

Static methods with no side effects are faster and more testable.

// DO: Static pure function public static class OrderCalculator { public static Money CalculateTotal(IReadOnlyList<OrderItem> items) { var total = items.Sum(i => i.Price * i.Quantity); return new Money(total, "USD"); } }

// Usage - predictable, testable var total = OrderCalculator.CalculateTotal(items);

Benefits:

  • No vtable lookup (faster)

  • No hidden state

  • Easier to test (pure input → output)

  • Thread-safe by design

  • Forces explicit dependencies

// DON'T: Instance method hiding dependencies public class OrderCalculator { private readonly ITaxService _taxService; // Hidden dependency private readonly IDiscountService _discountService; // Hidden dependency

public Money CalculateTotal(IReadOnlyList&#x3C;OrderItem> items)
{
    // What does this actually depend on?
}

}

// BETTER: Explicit dependencies via parameters public static class OrderCalculator { public static Money CalculateTotal( IReadOnlyList<OrderItem> items, decimal taxRate, decimal discountPercent) { // All inputs visible } }

Don't go overboard - Use instance methods when you genuinely need state or polymorphism.

Defer Enumeration

Don't materialize enumerables until necessary. Avoid excessive LINQ chains.

// BAD: Premature materialization public IReadOnlyList<Order> GetActiveOrders() { return _orders .Where(o => o.IsActive) .ToList() // Materialized! .OrderBy(o => o.CreatedAt) // Another iteration .ToList(); // Materialized again! }

// GOOD: Defer until the end public IReadOnlyList<Order> GetActiveOrders() { return _orders .Where(o => o.IsActive) .OrderBy(o => o.CreatedAt) .ToList(); // Single materialization }

// GOOD: Return IEnumerable if caller might not need all items public IEnumerable<Order> GetActiveOrders() { return _orders .Where(o => o.IsActive) .OrderBy(o => o.CreatedAt); // Caller decides when to materialize }

Async Enumeration

Be careful with async and IEnumerable:

// BAD: Async in LINQ - hidden allocations var results = orders .Select(async o => await ProcessOrderAsync(o)) // Task per item! .ToList(); await Task.WhenAll(results);

// GOOD: Use IAsyncEnumerable for streaming public async IAsyncEnumerable<OrderResult> ProcessOrdersAsync( IEnumerable<Order> orders, [EnumeratorCancellation] CancellationToken ct = default) { foreach (var order in orders) { ct.ThrowIfCancellationRequested(); yield return await ProcessOrderAsync(order, ct); } }

// GOOD: Batch processing for parallelism var results = await Task.WhenAll( orders.Select(o => ProcessOrderAsync(o)));

ValueTask vs Task

Use ValueTask for hot paths that often complete synchronously. For real I/O, just use Task .

// DO: ValueTask for cached/synchronous paths public ValueTask<User?> GetUserAsync(UserId id) { if (_cache.TryGetValue(id, out var user)) { return ValueTask.FromResult<User?>(user); // No allocation }

return new ValueTask&#x3C;User?>(FetchUserAsync(id));

}

// DO: Task for real I/O (simpler, no footguns) public Task<Order> CreateOrderAsync(CreateOrderCommand cmd) { // This always hits the database return _repository.CreateAsync(cmd); }

ValueTask rules:

  • Never await a ValueTask more than once

  • Never use .Result or .GetAwaiter().GetResult() before completion

  • If in doubt, use Task

Span and Memory for Bytes

Use Span<T> and Memory<T> instead of byte[] for low-level operations.

// DO: Accept Span for synchronous operations public static int ParseInt(ReadOnlySpan<char> text) { return int.Parse(text); }

// DO: Accept Memory for async operations public async Task WriteAsync(ReadOnlyMemory<byte> data) { await _stream.WriteAsync(data); }

// DON'T: Force array allocation public static int ParseInt(string text) // String allocated { return int.Parse(text); }

Common Span Patterns

// Slice without allocation ReadOnlySpan<char> span = "Hello, World!".AsSpan(); var hello = span[..5]; // No allocation

// Stack allocation for small buffers Span<byte> buffer = stackalloc byte[256];

// Use ArrayPool for larger buffers var buffer = ArrayPool<byte>.Shared.Rent(4096); try { // Use buffer... } finally { ArrayPool<byte>.Shared.Return(buffer); }

Collection Return Types

Return Immutable Collections from APIs

// DO: Return immutable collection public IReadOnlyList<Order> GetOrders() { return _orders.ToList(); // Caller can't modify internal state }

// DO: Use frozen collections for static data (.NET 8+) private static readonly FrozenDictionary<string, Handler> _handlers = new Dictionary<string, Handler> { ["create"] = new CreateHandler(), ["update"] = new UpdateHandler(), }.ToFrozenDictionary();

// DON'T: Return mutable collection public List<Order> GetOrders() { return _orders; // Caller can modify! }

Internal Mutation is Fine

public IReadOnlyList<OrderItem> BuildOrderItems(Cart cart) { var items = new List<OrderItem>(); // Mutable internally

foreach (var cartItem in cart.Items)
{
    items.Add(CreateOrderItem(cartItem));
}

return items;  // Return as IReadOnlyList

}

Collection Guidelines

Scenario Return Type

API boundary IReadOnlyList<T> , IReadOnlyCollection<T>

Static lookup data FrozenDictionary<K,V> , FrozenSet<T>

Internal building List<T> , then return as readonly

Single item or none T? (nullable)

Zero or more, lazy IEnumerable<T>

Quick Reference

Pattern Benefit

sealed class

Devirtualization, clear API

readonly record struct

No defensive copies, value semantics

Static pure functions No vtable, testable, thread-safe

Defer .ToList()

Single materialization

ValueTask for hot paths Avoid Task allocation

Span<T> for bytes Stack allocation, no copying

IReadOnlyList<T> return Immutable API contract

FrozenDictionary

Fastest lookup for static data

Anti-Patterns

// DON'T: Unsealed class without reason public class OrderService { } // Seal it!

// DON'T: Mutable struct public struct Point { public int X; public int Y; } // Make readonly

// DON'T: Instance method that could be static public int Add(int a, int b) => a + b; // Make static

// DON'T: Multiple ToList() calls items.Where(...).ToList().OrderBy(...).ToList(); // One ToList at end

// DON'T: Return List<T> from public API public List<Order> GetOrders(); // Return IReadOnlyList<T>

// DON'T: ValueTask for always-async operations public ValueTask<Order> CreateOrderAsync(); // Just use Task

Resources

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