type-design-performance

name	type-design-performance
description	Design .NET types for performance. Covers struct vs class decision matrix, sealed by default, readonly structs, ref struct and Span/Memory selection, FrozenDictionary, ValueTask, and collection return types. Use when designing new types and APIs, reviewing code for performance issues, choosing between class, struct, and record, or working with collections and enumerables.

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

1. Seal your types - Unless explicitly designed for inheritance
2. Prefer readonly structs - For small, immutable value types
3. Prefer static pure functions - Better performance and testability
4. Defer enumeration - Don't materialize until you need to
5. 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:

1. JIT devirtualization -- sealed classes enable the JIT to replace virtual calls with direct calls, enabling inlining
2. Simpler contracts -- unsealed classes imply a promise to support inheritance
3. 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<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<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

1. Do not default to class for every type -- evaluate the struct vs class decision matrix.
2. Do not create non-readonly structs -- mutable structs cause subtle bugs.
3. Do not use Span<T> in async methods -- use Memory<T> for async code.
4. Do not use FrozenDictionary for mutable data -- it has no add/remove APIs.
5. Do not seal abstract classes or classes designed as extension points.
6. Do not make large structs (> 64 bytes) without measuring -- benchmark copy cost.
7. Do not use Dictionary<K,V> for static lookup tables in hot paths -- use FrozenDictionary.
8. Do not forget in parameter for large readonly structs -- without in, the struct is copied.

---

Resources

• Performance Best Practices: https://learn.microsoft.com/en-us/dotnet/standard/performance/
• Span Guidance: https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/
• Frozen Collections: https://learn.microsoft.com/en-us/dotnet/api/system.collections.frozen
• Framework Design Guidelines: Type Design: https://learn.microsoft.com/dotnet/standard/design-guidelines/type
• Choosing between class and struct: https://learn.microsoft.com/dotnet/standard/design-guidelines/choosing-between-class-and-struct
• ref struct types: https://learn.microsoft.com/dotnet/csharp/language-reference/builtin-types/ref-struct
• Records (C# reference): https://learn.microsoft.com/dotnet/csharp/language-reference/builtin-types/record