type-design-performance

Type Design for Performance

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Install skill "type-design-performance" with this command: npx skills add wshaddix/dotnet-skills/wshaddix-dotnet-skills-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

Struct vs Class Decision Matrix

Choosing between struct and class at design time has cascading effects on allocation, GC pressure, and API shape.

Decision Criteria

Criterion Favors struct

Favors class

Size Small (<= 16 bytes ideal, <= 64 bytes acceptable) Large or variable size

Lifetime Short-lived, method-scoped Long-lived, shared across scopes

Identity Value equality (two instances with same data are equal) Reference identity matters

Mutability Immutable (readonly struct ) Mutable or complex state transitions

Inheritance Not needed Requires polymorphism or base class

Nullable semantics default is a valid zero state Needs explicit null to signal absence

Collection usage Stored in arrays/spans (contiguous memory) Stored via references (indirection)

Size Guidelines

<= 16 bytes: Ideal struct -- fits in two registers, passed efficiently 17-64 bytes: Acceptable struct -- measure copy cost vs allocation cost

64 bytes: Prefer class -- copying cost outweighs allocation avoidance

Common Types and Their Correct Design

Type Correct Choice Why

Point2D (8 bytes: two floats) readonly struct

Small, immutable, value semantics

Money (16 bytes: decimal + currency) readonly struct

Small, immutable, value equality

DateRange (16 bytes: two DateOnly) readonly struct

Small, immutable, value semantics

Matrix4x4 (64 bytes: 16 floats) struct (with in parameters) Performance-critical math

CustomerDto (variable: strings, lists) class or record

Contains references, variable size

HttpRequest context class

Long-lived, shared across middleware

Sealed by Default

Why Seal Library Types

For library types (code consumed by other assemblies), seal classes by default:

  • JIT devirtualization -- sealed classes enable the JIT to replace virtual calls with direct calls, enabling inlining

  • Simpler contracts -- unsealed classes imply a promise to support inheritance

  • Fewer breaking changes -- sealing a class later is a binary-breaking change

// GOOD -- sealed by default for library types public sealed class WidgetService { public Widget GetWidget(int id) => new(id, "Default"); }

// Only unseal when inheritance is an intentional design decision public abstract class WidgetValidatorBase { public abstract bool Validate(Widget widget); protected virtual void OnValidationComplete(Widget widget) { } }

When NOT to Seal

Scenario Reason

Abstract base classes Inheritance is the purpose

Framework extensibility points Consumers need to subclass

Test doubles in non-mockable designs Mocking frameworks need to subclass

Application-internal classes Sealing adds no value

Readonly Structs

Mark structs readonly when all fields are immutable. This eliminates defensive copies the JIT creates when accessing structs through in parameters or readonly fields.

The Defensive Copy Problem

// NON-readonly struct -- JIT must defensively copy on every method call public struct MutablePoint { public double X; public double Y; public double Length() => Math.Sqrt(X * X + Y * Y); }

public double GetLength(in MutablePoint point) { return point.Length(); // Hidden copy here! }

// GOOD -- readonly struct: JIT knows no mutation is possible public readonly struct ImmutablePoint { public double X { get; } public double Y { get; }

public ImmutablePoint(double x, double y) => (X, Y) = (x, y);

public double Length() => Math.Sqrt(X * X + Y * Y);

}

public double GetLength(in ImmutablePoint point) { return point.Length(); // No copy, direct call }

Readonly Struct Checklist

  • All fields are readonly or { get; } / { get; init; } properties

  • No methods mutate state

  • Constructor initializes all fields

  • Consider IEquatable<T> for value comparison without boxing

Record Types for Data Transfer

record class vs record struct

Characteristic record class

record struct

Allocation Heap Stack (or inline in arrays)

Equality Reference type with value equality Value type with value equality

with expression Creates new heap object Creates new stack copy

Nullable null represents absence default represents empty state

Size Reference (8 bytes on x64) + heap Full size on stack

// record class -- heap allocated, good for DTOs public record CustomerDto(string Name, string Email, DateOnly JoinDate);

// readonly record struct -- stack allocated, good for small value objects public readonly record struct Money(decimal Amount, string Currency);

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

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); }

Async Enumeration

// 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) { 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

ref struct and Span/Memory Selection

ref struct Constraints

ref struct types are stack-only: they cannot be boxed, stored in fields of non-ref-struct types, or used in async methods.

Span vs Memory Decision

Criterion Use Span<T>

Use Memory<T>

Synchronous method Yes Yes (but Span is lower overhead)

Async method No (ref struct) Yes

Store in field/collection No (ref struct) Yes

Pass to callback/delegate No Yes

Slice without allocation Yes Yes

Wrap stackalloc buffer Yes No

Selection Flowchart

Will the buffer be used in an async method or stored in a field? YES -> Use Memory<T> (convert to Span<T> with .Span for synchronous processing) NO -> Do you need to wrap a stackalloc buffer? YES -> Use Span<T> NO -> Prefer Span<T> for lowest overhead

Practical Pattern

// Public API uses Memory<T> for maximum flexibility public async Task<int> ProcessAsync(ReadOnlyMemory<byte> data, CancellationToken ct = default) { await _stream.WriteAsync(data, ct); return CountNonZero(data.Span); }

// Internal hot-path method uses Span<T> for zero overhead private static int CountNonZero(ReadOnlySpan<byte> data) { var count = 0; foreach (var b in data) { if (b != 0) count++; } return count; }

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 Type Selection

Decision Matrix

Scenario Recommended Type Rationale

Build once, read many FrozenDictionary<K,V> / FrozenSet<T>

Optimized read layout (.NET 8+)

Build once, read many (pre-.NET 8) ImmutableDictionary<K,V>

Thread-safe, immutable

Concurrent read/write ConcurrentDictionary<K,V>

Thread-safe without external locking

Frequent modifications Dictionary<K,V>

Lowest per-operation overhead

Ordered data SortedDictionary<K,V>

O(log n) lookup with sorted enumeration

Return from public API IReadOnlyList<T> / IReadOnlyDictionary<K,V>

Immutable interface

Stack-allocated small collection Span<T> with stackalloc Zero GC pressure

FrozenDictionary (.NET 8+)

FrozenDictionary<K,V> optimizes the internal layout at creation time for maximum read performance:

using System.Collections.Frozen;

private static readonly FrozenDictionary<string, int> StatusCodes = new Dictionary<string, int> { ["OK"] = 200, ["NotFound"] = 404, ["InternalServerError"] = 500 }.ToFrozenDictionary(StringComparer.OrdinalIgnoreCase);

public int GetStatusCode(string name) => StatusCodes.TryGetValue(name, out var code) ? code : -1;

When to use FrozenDictionary:

  • Configuration lookup tables populated at startup

  • Static mappings (enum-to-string, error codes, feature flags)

  • Any dictionary populated once and read many times

When NOT to use:

  • Data that changes at runtime

  • Small lookups (< 10 items) where optimization overhead is not recouped

Collection Return Types

// DO: Return immutable collection public IReadOnlyList<Order> GetOrders() { return _orders.ToList(); }

// DO: Use frozen collections for static data 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! }

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

// DON'T: Use Span&#x3C;T> in async methods public async Task ProcessAsync(Span<byte> data); // Use Memory<T>

// DON'T: Use FrozenDictionary for mutable data // It has no add/remove APIs

Agent Gotchas

  • Do not default to class for every type -- evaluate the struct vs class decision matrix.

  • Do not create non-readonly structs -- mutable structs cause subtle bugs.

  • Do not use Span<T> in async methods -- use Memory<T> for async code.

  • Do not use FrozenDictionary for mutable data -- it has no add/remove APIs.

  • Do not seal abstract classes or classes designed as extension points.

  • Do not make large structs (> 64 bytes) without measuring -- benchmark copy cost.

  • Do not use Dictionary<K,V> for static lookup tables in hot paths -- use FrozenDictionary .

  • Do not forget in parameter for large readonly structs -- without in , the struct is copied.

Resources

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