type-design-performance

1# Type Design for Performance
2 
3## When to Use This Skill
4 
5Use this skill when:
6- Designing new types and APIs
7- Reviewing code for performance issues
8- Choosing between class, struct, and record
9- Working with collections and enumerables
10 
11---
12 
13## Core Principles
14 
151. **Seal your types** - Unless explicitly designed for inheritance
162. **Prefer readonly structs** - For small, immutable value types
173. **Prefer static pure functions** - Better performance and testability
184. **Defer enumeration** - Don't materialize until you need to
195. **Return immutable collections** - From API boundaries
20 
21---
22 
23## Seal Classes by Default
24 
25Sealing classes enables JIT devirtualization and communicates API intent.
26 
27```csharp
28// DO: Seal classes not designed for inheritance
29public sealed class OrderProcessor
30{
31    public void Process(Order order) { }
32}
33 
34// DO: Seal records (they're classes)
35public sealed record OrderCreated(OrderId Id, CustomerId CustomerId);
36 
37// DON'T: Leave unsealed without reason
38public class OrderProcessor  // Can be subclassed - intentional?
39{
40    public virtual void Process(Order order) { }  // Virtual = slower
41}
42```
43 
44**Benefits:**
45- JIT can devirtualize method calls
46- Communicates "this is not an extension point"
47- Prevents accidental breaking changes
48 
49---
50 
51## Readonly Structs for Value Types
52 
53Structs should be `readonly` when immutable. This prevents defensive copies.
54 
55```csharp
56// DO: Readonly struct for immutable value types
57…